turbulent velocity profiles for smooth and rough open channel flow velocity measurements using a laserdoppler anemometer are carried out in a fully developed rectangular subcritical open channel flow on smooth and rough beds the average roughness heights of t

re os isotope systematics and weathering of precambrian crustal rocks implications for the marine osmium isotope record the effects of weathering of freshly exposed precambrian rocks on the mobility of osmium were studied on soils developed on a chronosequence of glacial moraines from the western wind river range in wyoming the os budget of the precambrian granitoid source rocks is dominated by os rich trace phases such as magnetite amongst the major silicates biotite or a trace phase in biotite carries most of the os and re and is also the most radiogenic mineral with 187os 186os of 113 and 187re 186os of 2 000 re os isotope systematics of source rocks and soils indicate that rapid oxidation of magnetite mobilizes os with an isotopic composition similar to the isotopic composition of the bulk soils a very radiogenic fraction of os is mobilized through preferential weathering of biotite radiogenic runoff from precambrian shields inferred from osmium isotope analyses of freshwater fe mn nodules suggests that high latitude precambrian shields are important source areas of radiogenic os in seawater we propose that glacial scouring and weathering of glacial tills exposed after deglaciation of precambrian shields surrounding the north atlantic provides a mechanism for the slightly more radiogenic nature of north atlantic seawater compared to other seawater masses glacial interglacial variations in the osmium isotopic composition of seawater seem plausible and may be triggered by changes in weathering regimes on glacial interglacial time scales in high latitude shield areas surrounding the north atlantic

a continuum mechanical formulation for shallow polythermal ice sheets this paper is concerned with a new theoretical approach to model grounded ice sheets in three dimensions these are considered as polythermal i e there will be regions with temperatures below the

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

a sea ice climate switch mechanism for the 100 kyr glacial cycles a box model of the ocean atmosphere sea ice land ice climate system is used to study a novel mechanism for self sustained oscillations of the climate system on a time scale of 100 000 years without external forcing the oscillation in land ice volume has the familiar sawtooth shape of climate proxy records the most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice the sea ice acts as a switch of the climate system switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillation s sawtooth structure a simple explanation of the 100 kyr timescale is formulated on the basis of the mechanism seen in the model finally rapid sea ice changes such as those seen in our model and their drastic effects on the climate system may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord

evidence for post metamorphic metasomatism of high grade orthogneisses from sri lanka abstract here we discuss the post metamorphic metasomatism of high grade orthogneisses by studying granite looking pink coloured microcline bearing rocks exposed around ambagaspitiya sri lanka these rocks are medium to coarse grained and are more or less homogeneous and isotropic textural and petrographic analyses clearly show that these special rocks are neither deformed nor metamorphosed and that they are not any kind of intrusive rocks the present study shows that these rocks have formed through k metasomatism of once intensely deformed and metamorphosed granodiorite tonalite monzodiorite and quartz monzodiorite the modal compositions of most of these metasomatic rocks of ambagaspitiya are very similar to those of syenite quartz syenite monzonite quartz monzonite and quartz monzodiorite all the original metamorphic rocks namely granitic gneiss metagranite metagranodiorite metatonalite metamonzodiorite metaquartz monzodiorite metadiorite basic dikes and metapelites had undergone at least five ductile deformations d 1 to d 5 and had been metamorphosed under upper amphibolite to granulite facies conditions prior to the metasomatism almost all the parent metamorphic rocks had acquired a well developed gneissic foliation s2 and had suffered at least two intense folding events f 3 and f 5 before the metasomatism occurred all the metamorphic and deformational fabrics of affected metamorphic rocks have been completely or partially obliterated by the metasomatism this indicates that the metasomatic process post dates all ductile deformations d 1 d 5 and the regional metamorphism of the parent metamorphic rocks metagranodiorite metatonalite metamonzodiorite and metaquartz monzodiorite have undergone intense metasomatism it is shown that the metasomatism has nucleated along late stage post d 5 shear zones which may form an interconnected network potassium bearing metasomatic fluids derived from a deep seated k rich source may have migrated along these shear zones the fluids which entered the shear zones have pervaded the orthogneisses through foliation planes and along grain boundaries and microcracks in minerals transforming the host gneisses to metasomatic rocks the main metasomatic transformation has taken place through the replacement of metamorphic plagioclase and plagioclase quartz by microcline and through formation of myrmekite further studies are necessary to unravel the nature composition and the source of these late stage k rich fluids in the lower crust

atlantic overturning responses to late pleistocene climate forcings variations in the earth s orbit are known to influence glacial cycles and consequently global ice volume and the strength of oceanic mixing sediment cores from the ocean floor are used to assess orbit climate interactions and contain a wealth of information on elemental ratios such as mg ca species assemblages and in particular isotopes of carbon used to approximate nutrient content and oxygen indicative of both global ice volume and deep ocean temperature established wisdom from the specmap project suggests that the three orbital cycles precession obliquity and ellipticity induce similar changes in climate and ocean circulation lisiecki et al reanalyse sediment cores and suggest a substantially different model for orbit climate interactions in the vast mid depth atlantic ocean peaks in solar radiation are associated with stronger circulation at the obliquity band and weaker circulation at the precession band these findings could lead to a substantial revision in our understanding of the forcings of glacial cycles and ocean circulation the factors driving glacial changes in ocean overturning circulation are not well understood on the basis of a comparison of 20 climate variables over the past four glacial cycles the specmap project1 proposed that summer insolation at high northern latitudes that is milankovitch forcing drives the same sequence of ocean circulation and other climate responses over 100 kyr eccentricity cycles 41 kyr obliquity cycles and 23 kyr precession cycles specmap analysed the circulation response at only a few sites in the atlantic ocean however and the phase of circulation response has been shown to vary by site and orbital band2 here we test the specmap hypothesis by measuring the phase of orbital responses in benthic 13c a proxy indicator of ocean nutrient content at 24 sites throughout the atlantic over the past 425 kyr on the basis of 13c responses at 3 000 4 010 m water depth we find that maxima in milankovitch forcing are associated with greater mid depth overturning in the obliquity band but less overturning in the precession band this suggests that atlantic overturning is strongly sensitive to factors beyond ice volume and summer insolation at high northern latitudes a better understanding of these processes could lead to improvements in model estimates of overturning rates which range from a 40 per cent increase to a 40 per cent decrease at the last glacial maximum3 and a 10 50 per cent decrease over the next 140 yr in response to projected increases in atmospheric co2 ref 4

an 800 kyr record of global surface ocean δ18o and implications for ice volume temperature coupling abstract the sequence of feedbacks that characterized 100 kyr glacial cycles of the past million years remains uncertain hampering an understanding of the interconnections between insolation ice sheets greenhouse gas forcing and climate critical to addressing this issue is an accurate interpretation of the marine 18 o record the main template for the ice ages this study uses a global compilation of 49 paired sea surface temperature planktonic 18 o records to extract the mean 18 o of surface ocean seawater over the past 800 kyr which we interpret to dominantly reflect global ice volume the results indicate that global surface temperature inferred deep ocean temperature and atmospheric co2 decrease early during each glacial cycle in close association with one another whereas major ice sheet growth occurs later in glacial cycles these relationships suggest that ice volume may have exhibited a threshold response to global cooling and that global deglaciations do not occur until after the growth of large ice sheets this phase sequence also suggests that the ice sheets had relatively little feedback on global cooling simple modeling shows that the rate of ice volume change through time is largely determined by the combined influence of insolation temperature and ice sheet size with possible implications for the evolution of glacial cycles over the past three million years

velocity distributions in spatially varied flow with increasing discharge the experimental study presented in this paper explores the distribution of the mean velocities for a channel receiving spatially varied sv inflow from directly above the centerline of the channel via sets of nozzles the velocity components u v and w have been measured using a laser doppler velocimeter initially the validity of the law of the wall in the channel receiving sv inflow was investigated it was found that the measured velocity profile within the inertial sublayer region near the channel bed falls below the line representing the log law further if the sv inflow enters at the center of the channel the degree that the data depart from the log law increases from the sidewall toward the center of the channel the structures of the velocity distributions are also presented in detail the results include both near field that is the area close to where the flow enters the channel and far field measurements it was found that the influence of the sv inflow on velocity profiles is largely c

rb sr and sm nd geochronology of the eppawala metamorphic rocks and carbonatite wanni complex sri lanka abstract petrological studies on the surrounding metamorphic rocks of the eppawala carbonatite body wanni complex sri lanka revealed that these rocks had been metamorphosed under amphibolite to granulite facies conditions garnet sillimanite biotite gneiss shows lower range of metamorphic temperature 730 770 c than the migmatite gneiss 750 780 c and the pressure varies from 6 6 7 8 kbar to 5 6 6 4 kbar respectively the metamorphic age of the garnet sillimanite biotite gneiss and migmatite gneiss dated 607 23 ma and 626 16 ma respectively for mineral whole rock isochron in sm nd system these ages are compatible with the ages of regional high grade metamorphism occurred 610 550 ma in the three crustal units in sri lanka rb sr system for biotite apatite and whole rock fractions suggests 493 5 ma for the eppawala carbonatite body this age indicates the cooling age of the biotite the presence of non crystalline carbonatite matrix and large hexagonal apatite crystals suggests a slow cooling history further low closure temperature of biotite in rb sr system suggests that the intrusion age of carbonatite body should be more than 493 ma but non metamorphosed nature provides evidence that the intrusion age of the carbonatite body should be less than the period of regional metamorphism 610 550 ma therefore eppawala carbonatite body has a strong possibility to be a late to post magmatic intrusion the other late to post magmatic intrusions in the wanni complex and highland complex are dated between 580 550 ma therefore the most probable intrusion age of the eppawala carbonatite body is suggested to be around 550 ma

evidence for a fold nappe in the high grade basement of central sri lanka terrane assembly in the pan african lower crust abstract structural analysis in the kadugannawa complex kc in central sri lanka reveals the presence of an early steep gneissic layering s 1 which has been overprinted by a younger event d 2 in the structurally deepest levels d 2 folds become progressively tighter towards the contact with the underlying highland complex hc the distribution of lithologies in the kc the distribution of steep s 1 and subhorizontal s 2 and the strain gradient towards the kc hc contact suggest that the kc is a d 2 fold nappe overlying the hc vergence of asymmetric sheath folds in sections parallel to a strong nnw sse lineation is towards nnw indicating a top to the nnw displacement granulite facies assemblages were formed during and after the final stages of d 2 in the kc d 2 was the main fabric forming event in the hc where meta igneous and metasedimentary gneisses show l s fabrics defined by the peak granulite facies assemblages the sense of shear deduced from asymmetric foliation boudinage is consistent with the top to the nnw displacement in the kc both units have been refolded around large scale upright d 3 folds the nappe model and nd model ages suggest that the kc and the eastern part of the hc may represent distinct lower crustal terranes juxtaposed during pan african high grade high strain events d 2 the contact between them is interpreted as a terrane boundary in a lower crustal environment the top to the nnw movement sense suggests that the kc may have been originally located south of the eastern highland complex this and the similarity in isotope signatures indicate a pre assembly link of the kc the western highland complex and the vijayan complex of eastern sri lanka

measurement of local bed shear stress in streams using a preston static tube local bed shear stress t w is a fluid dynamic parameter of importance in determining the physical and biological characteristics of stream bed environments unfortunately it is often difficult to measure t w under field conditions herein we describe the use of a preston static tube which is essentially a surface mounted pitot static tube to measure t w in mountain streams our results indicate that it is possible to measure local shear stress quickly consistently and inexpensively in the field this technique also provides high spatial resolution which should allow for detailed in situ studies of local shear stress at scales relevant to lotic organisms such information will be invaluable in studies of benthic organisms and hydraulically relevant phenomena in the near bottom zone of lotic systems

modeling north american freshwater runoff through the last glacial cycle the northern hemisphere ice sheets decayed rapidly during deglacial phases of the ice age cycle producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation the continental record of ice sheet history is more obscured during the growth and advance of the last great ice sheets ca 120 000 20 000 yr b p but ice cores tell of high amplitude millennial scale climate fluctuations that prevailed throughout this period these climatic excursions would have provoked significant fluctuation of ice sheet margins and runoff variability whenever ice sheets extended to mid latitudes giving a complex pattern of freshwater delivery to the oceans a model of continental surface hydrology is coupled with an ice dynamics model simulating the last glacial cycle in north america meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice sheet margin the coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins results imply an intensified surface hydrological environment when ice sheets are present despite a net decrease in precipitation during glacial periods diminished continental evaporation and high levels of meltwater production combine to give mid latitude runoff values that are highly variable through the glacial cycle but are two to three times in excess of modern river fluxes drainage to the north atlantic via the st lawrence hudson and mississippi river catchments averages 0 356 sv for the period 60 000 10 000 yr b p compared to 0 122 sv for the past 10 000 yr high amplitude meltwater pulses to the gulf of mexico north atlantic and north pacific occur throughout the glacial period with ice sheet geometry controlling intricate patterns of freshwater routing variability runoff from north america is staged in the final deglaciation with a stepped sequence of pulses through the mississippi st lawrence arctic and hudson strait drainages

changes in the silt and clay size mineralogy of sediments at ocean drilling program site 645b baffin bay site 645b ocean drilling program odp leg 105 was cored in 2010 m water depth off the eastern margin of baffin island a 279 m core was dated as extending back to ca 2 ma samples of sediment were analyzed for their silt and clay size mineralogy using x ray diffraction techniques constrained cluster analysis on these data defined three units which indicated that significant changes in mineralogies occurred at approximately 75 m below sea floor bsf ca 0 5 ma and 155 m bsf ca 1 15 ma these boundaries coincide closely with units ia ib and ib ii as designated in the odp shipboard reports and which were based on detrital carbonate and sand contents unit ii 1 2 2 ma is characterized by zero clay size calcite and high amounts of clay size smectite and silt size mica unit ib 0 5 1 15 ma is noteworthy for the first presence of clay size calcite high amounts of detrital dolomite and a peak in clay size kaolinite the uppermost unit ia 0 0 5 ma is characterized by consistently high value

influence of high latitude vegetation feedbacks on late palaeozoic glacial cycles ice ages during the palaeozoic era are marked by glacial interglacial cycles thought to be driven by variations in the earth s orbit numerical simulations suggest that the response of vegetation to the varying insolation may be an important factor in the associated climate response

boundary shear stress in spatially varied flow with increasing discharge the distribution of the wall shear stress on the bed and sidewalls of an open channel receiving lateral inflow was obtained from experimental measurements of the distribution of the velocity in the viscous sublayer using a laser doppler velocimeter the experiments were conducted in a 0 4 m wide by 7 5 m long flume lateral inflow was provided into the channel from above via sets of nozzles positioned toward the downstream end of the flume lateral inflow was provided over a length of 1 9 m the results indicate that the local boundary shear stresses are significantly influenced by lateral inflow the significant variation occurs near and around the region where the lateral inflow enters the channel at various measurement positions along the lateral inflow zone mean boundary mean wall and mean bed shear stresses were obtained and compared the results indicate that the mean boundary shear stresses increase from the upstream to the downstream ends of the lateral inflow zone the results also indicate t

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

how obliquity cycles powered early pleistocene global ice volume variability milankovitch theory proposes that the magnitude of high latitude summer insolation dictates the continental ice volume response by controlling summer snow melt thus anticipating a substantial ice volume contribution from the strong summer insolation signal of precession yet almost all of the early pleistocene 18o records signal strength resides at the frequency of obliquity here we explore this discrepancy using a climate vegetation ice sheet model to simulate climate ice sheet response to transient orbits of varying obliquity and precession spectral analysis of our results shows that despite contributing significantly less to the summer insolation signal almost 60 of the ice volume power exists at the frequency of obliquity due to a combination of albedo feedbacks seasonal offsets and orbital cycle duration differences including eccentricity modulation of the precession ice volume component and assuming a small antarctic ice response to orbital forcing produce a signal that agrees with the 18o ice volume proxy records

zircon ion microprobe dating of high grade rocks in sri lanka the high grade gneisses of sri lanka display spectacular in situ granulitization phenomena similar to those observed in southern india and of current interest for evolutionary models of the lower continental crust the absolute ages of these rocks are poorly constrained and so using the shrimp ion microprobe we have analyzed small spots on zircons from upper amphibolite to granulite grade quartzitic and pelitic metasediments detrital grains from a metaquartzite of the highland group preserve premetamorphic u pb ages of between 3 17 and 2 4 ga and indicate derivation of the sediment from an unidentified archean source terrain the pb loss patterns of these zircons and the other samples suggest severe disturbance at ca 1100 ma ago which we attribute to high grade regional metamorphism two pelitic gneisses contain detrital zircons with ages up to 2 04 ga and also record an approx 1100 ma event that is also apparent from metamorphic rims around old cores and new zircon growth a granite intrusive into

sensitivity of paleoclimate simulation results to season definitions according to the milankovitch theory slow variations of the earth s orbital parameters change the amplitude of the seasonal cycle of insolation and are considered to be the main forcing mechanism of glacial interglacial cycles because of the precession and changes in eccentricity the length of seasons also varies no absolute phasing is then possible between the insolation curves of two different periods various solutions to compare different periods have been given either for astronomical computations e g berger and loutre 1991 laskar et al 1993 or for model simulations e g kutzbach and otto bliesner 1982 mitchell et al 1988 but the sensitivity of model results to the different possible solutions has never been quantified our results based on simulations of the last interglacial climate 126 kyr b p where changes in the length of the seasons are large clearly show that phase leads or lags between the various solutions used introduce biases in the analysis of insolation and climate change of the same order of magnitude as the milankovitch forcing our main conclusions are that 1 when comparing various model simulations the date of the vernal equinox i e the phasing of the seasonal cycle of insolation as well as the definition of seasons must be the same for all models in order to avoid artificial differences 2 seasons based on astronomical positions are preferred to seasons defined with the same lengths as today since they better account for the phasing of insolation curves however insolation is not the only forcing in most atmospheric general circulation model simulations we also discuss the impact of the calendar hidden behind the definition of the seasonal cycle of the other boundary conditions such as sea ice or sea surface temperatures

plio pleistocene climate evolution trends and transitions in glacial cycle dynamics abstract we describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5 3 myr in particular we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic 18 o when the exponential trend in 18 o variance is removed its spectral power exhibits strong proportional responses to amplitude modulations in orbital forcing over most of the plio pleistocene precession responses correlate with modulations in forcing for the last 5 myr but 41 kyr response is sensitive to obliquity modulation only before 1 4 myr where responses are sensitive to modulations in forcing we demonstrate that glacial cycles are orbitally forced rather than being self sustained or paced by orbital changes the shapes of glacial cycles have several nonlinear properties which may be indicative of glacial interglacial differences in climate sensitivity or response time the saw tooth asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation and the relative duration of interglacial stages decreases at 1 4 myr collectively trends in the shape of glacial cycles and the sensitivity of 18 o to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2 5 and 1 4 myr but show no significant change associated with the appearance of strong 100 kyr cycles during the mid pleistocene transition

integrated summer insolation forcing and 40 000 year glacial cycles the perspective from an ice sheet energy balance model although the origins of the 40 000 year glacial cycles during the early pleistocene are readily attributed to changes in earth s obliquity also having a 40 000 year period the lack of ice volume variability at precession periods 20 000 years is difficult to reconcile with most parameterizations of the insolation forcing it was recently proposed that precession s influence on glaciation is muted because variations in the intensity of summer insolation are counterbalanced by changes in the duration of the summertime but no climate model has yet been shown to generate obliquity period glacial cycles in response to the seasonal insolation forcing here we present a coupled ice sheet energy balance model that reproduces the seasonal cycle and when run over long time periods generates glacial variability in response to changes in earth s orbital configuration the model is forced by the full seasonal cycle in insolation and its response can be understood within the context of the integrated summer insolation forcing the simple fact that obliquity s period is roughly twice as long as that of precession results in a larger amplitude glacial response to obliquity however for the model to generate almost exclusively obliquity period glacial variability two other conditions must be met first the ice sheet s ablation zone must reside poleward of 60 n because insolation intensity is more sensitive to changes in earth s obliquity at high latitudes second the ablation season must be long enough for precession s opposing influences on summer and fall insolation intensity to counterbalance one another these conditions are consistent with a warm climate and a thin ice sheet where the latter is simulated as a response to subglacial sediment deformation if a colder climate is prescribed or in the absence of basal motion ice sheets tend to be larger and undergo greater precession period variability in keeping with proxy observations of late pleistocene glaciation

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

a layered basic intrusion deformed and metamorphosed in granulite facies of the sri lanka basement a layered basic intrusion has been found in the central granulite belt of the sri lanka continental basement it intruded parallel to bedding before all or early during deformation of neighbouring metasediments deformation affecting metasediments and the intrusion alike includes flattening to c 1 20 of the original thickness and nnw stretching to c 20 times the original length the intrusion is now 170 300 m thick most of the deformation was acquired under granulite facies metamorphism the intrusion was then folded still at high t by a large f4 synform with an axis parallel to str1 and a steep axial plane a steep axial plane cleavage and minor folds are related to this big fold stretching continued along its axis late during formation of this fold a granite intruded mainly following s4 cleavage planes the intrusion shows a homogeneous gabbroic series at the bottom followed upwards by a differentiated and layered series a thin sequence of ultramafic rocks occurs near the middle this indicates multiple melt injection more homogeneous partly biotite bearing amphibolites form the top of the succession magmatic layering is well preserved but no magmatic minerals or grain fabrics have escaped deformation or metamorphism static annealing under granulite facies conditions outlasted all deformation and was accompanied and followed by the beginning of cooling hornblende plagioclase coronas formed round garnets at this stage geochemical work carried out by stosch 1991 on our samples confirms the cumulate nature of the rocks

orbital tuning chronology for the vostok climate record supported by trapped gas composition abstract we present data on the o2 n2 ratios of trapped gas samples for the entire length of the vostok climate record as in other cores o2 n2 ratios in these samples are less than the atmospheric ratio by a small and variable amount because o2 is selectively excluded during the gas trapping process and because o2 is also preferentially lost in poorly preserved core samples samples younger than 150 ka have large and variable o2 depletions samples older than 200 ka have o2 n2 ratios that replicate well and vary smoothly with depth we plot o2 n2 ratios of well replicated samples older than 160 ka using a chronology derived by matching the 18o of paleoatmospheric o2 18oatm to northern hemisphere june insolation on this timescale o2 n2 varies coherently with local 78 s summertime insolation based on time series analysis of the o2 n2 record and the dynamics of snow metamorphism at the surface we conclude that summertime insolation influences physical properties of ice grains that control the degree of o2 exclusion during bubble closeoff o2 n2 in vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core we show that the 18oatm chronology supported by the coincidence of o2 n2 ratios with insolation is also compatible with recent radiometric dating of corals from high sea stands we further successfully test the 18oatm tuning chronology by showing that it predicts a chronology for the gisp2 core which is essentially indistinguishable from the standard gisp2 chronology and therefore in excellent agreement with the radiometric chronology of hulu cave china an accurate chronology for the vostok ice core is now in place

ice sheet erosion and the stripping of tertiary regolith from baffin island eastern canadian arctic abstract the erosion of glaciated regions and concomitant changes in ice sheet dynamics through the pleistocene are poorly documented the baffin island landscape which has been shaped by the foxe sector of the laurentide ice sheet lis contains a variety of glacial and proximal glaciomarine sedimentary archives spanning the pleistocene we examine these records to better understand when tertiary regolith was stripped from beneath the foxe sector of the lis till on the interior plateaux of the island in areas with scoured bedrock have low chemical index of alteration cia values low meteoric 10 be 10 be met concentrations and clay mineralogy consistent with erosion from an unweathered bedrock source however in some areas in between fiord onset zones more weathered till is present containing considerably higher cia values high 10 be met concentrations and secondary clay mineral weathering products implying that the till has persisted on the landscape and weathered during successive glaciations using these weathering signatures we analyze the coastal glaciogenic deposits of the clyde foreland formation cff at two sites for evidence of tertiary regolith removal from the interior of baffin island by lis erosion provenance indicators within the cff demonstrate that pleistocene lis ice flow lines across baffin island have remained generally constant the oldest cff glaciogenic unit likely representing one of the first if not the first lis advances across baffin island had high 10 be met concentrations at the time of deposition consistent with extensive regolith erosion evidence of notably weathered sediment is absent in all younger units suggesting that tertiary regolith was likely largely stripped from the interior of the island by 1 6 0 2 ma

terminating the last interglacial the role of ice sheet climate feedbacks in a gcm asynchronously coupled to an ice sheet model climatic deterioration in northeastern canada following the last interglacial resulted in the formation and abrupt expansion of the laurentide ice sheet however the physical mechanisms leading to rapid ice sheet expansion are not well understood here the authors report on experiments using an ice sheet model asynchronously coupled to a gcm to investigate the role of ice sheet climate feedbacks in terminating the last interglacial period in agreement with simpler models the experiments indicate that a specific type of ice albedo feedback the small ice cap instability is the dominant process controlling rapid expansion of the laurentide ice sheet as ice elevations increase in northeastern canada a stationary wave forms and strengthens over the laurentide ice sheet which acts to hinder further expansion of the ice margin and reduce the effect of the small ice cap instability the sensitivity of these feedbacks to ice topography results in a reduction in simulated ice volume when the communication interval between the gcm and ice sheet model is lengthened since this permits larger gains in ice elevation between gcm updates and biases the simulationtowardastrongerstationarywavefeedback theshortestcommunicationinterval 500 yr leadsto a laurentide ice volume of 6 3 10 6 km 3 in 10 kyr which is less than ice volume estimates based on the geological record but is a substantial improvement over previous gcm studies the authors discuss potential improvements to the asynchronous coupling scheme that would more accurately resolve ice sheet climate feedbacks potentially leading to greater simulated ice volume

age nd hf isotopes and geochemistry of the vijayan complex of eastern and southern sri lanka a grenville age magmatic arc of unknown derivation abstract the ca 1 0 1 1 ga vijayan complex vc of eastern and southeastern sri lanka is one of three high grade metamorphic terranes making up the basement of the island and is in tectonic contact with the adjacent older highland complex it consists predominantly of granitoid gneisses ranging in composition from diorite to leucogranite with a distinct calc alkaline geochemical signature and is interpreted as a magmatic arc strong ductile deformation has obliterated almost all original intrusive relationships high grade metamorphism during the pan african event at ca 610 520 ma has produced widespread granulite facies assemblages that are now largely retrogressed and were affected by extensive late metamorphic k metasomatism in southern sri lanka the vijayan gneisses are tectonically interlayered with rocks of the highland complex in a so called mixed zone we report zircon ages whole rock nd and hf in zircon isotopic systematics and geochemical data for a large selection of vijayan gneissses in order to better characterize this complex and its tectonic setting the zircon ages are predominantly in the range 1100 1000 ma with a few early neoproterozoic intrusions and identify the vc as a grenville age magmatic arc many zircons experienced minor to significant lead loss at about 580 ma the nd and hf isotopic data confirm a generally primitive origin for most vijayan gneisses but significant variations in both isotopic systems argue for source heterogeneities and the possible involvement of minor amounts of older continental material in their genesis immobile trace and rare earth element distributions favour an origin of the gneiss protoliths through melting at relatively shallow depth and the chemical variation from diorite and tonalite via granodiorite to granite in the vc can be explained by increasing fractionation of plagioclase and biotite accompanied by amphibole and accessory phases the vc may be comparable in some respect to the composition and evolution of the kohistan arc in pakistan the unique composition age and isotopic characteristics of the vc find no convincing counterpart in other fragments of the gondwana supercontinent and its origin therefore remains exotic and enigmatic

coupling of ice shelf melting and buttressing is a key process in ice sheets dynamics 1 increase in ice shelf melting is generally presumed to have triggered recent coastal ice sheet thinning using a full stokes finite element model which includes a proper description of the grounding line dynamics we investigate the impact of melting below ice shelves we argue that the influence of ice shelf melting on the ice sheet dynamics induces a complex response and the first naive view that melting inevitably leads to loss of grounded ice is erroneous we demonstrate that melting acts directly on the magnitude of the buttressing force by modifying both the area experiencing lateral resistance and the ice shelf velocity indicating that the decrease of back stress imposed by the ice shelf is the prevailing cause of inland dynamical thinning we further show that feedback from melting and buttressing forces can lead to nontrivial results as an increase in the average melt rate may lead to inland ice thickening and grounding line advance

velocity deformation model for unsteady open channel flows over smooth and rough beds abstracta one dimensional depth averaged velocity deformation model is developed to study the characteristics of the unsteady flows in open channels the vertical profile of the streamwise velocity is expressed in a power series and the coefficients of the velocity are evaluated by using the unsteady equation of motion a concise form of the friction velocity for unsteady nonuniform flow is then proposed by utilizing the coefficients of the power series the applicability of the model is validated with experimental data producing reasonably good agreement between the model results and the observed data finally the deformation of velocity at the surface is assured by comparing velocity distribution of numerical model with the computational results from a previous model

north american glacial meltwater history for the past 2 3 m y oxygen isotope evidence from the gulf of mexico a high resolution 18 o record from the northeastern gulf of mexico ocean drilling program leg 100 hole 625b provides a history of surface waters for the past 5 35 m y negative 18 o anomalies between 0 3 and 3 0 in planktic foraminifera beginning at 2 30 ma can only have resulted from the discharge of isotopically light glacial meltwater from the mississippi river meltwater anomalies implicate either the periodic presence of north american mid latitude glacial ice since the late pliocene or a drainage system very different from that of the present

bed shear from velocity profiles a new approach a new binary law of velocity distribution has been developed to describe the velocity profile for the entire flow region the law is a combination of logarithmic law valid in the wall inner region and parabolic law valid in the core outer region of the flow the validity of the law has been established based on earlier data on flat plates rough and smooth pipes and experimental data obtained from rigid walled open channels with plane sand beds a procedure of estimating bed shear stress from the proposed law of velocity distribution using the measured velocity profile has been evolved bed shear estimates made according to this procedure are in agreement with the values obtained from uniform flow analysis in the case of open channel flow over a sediment bed the proposed method of estimating the bed shear stress from the observed velocity profiles is found to be particularly useful in cases where it is difficult to determine precisely the true bed level such as in the case of flow over sediment beds

glacial variability over the last two million years an extended depth derived agemodel continuous obliquity pacing and the pleistocene progression abstract an agemodel not relying upon orbital assumptions is estimated over the last 2 ma using depth in marine sediment cores as a proxy for time agemodel uncertainty averages 10 ka in the early pleistocene 2 1 ma and 7 ka in the late pleistocene 1 ma to the present twelve benthic and five planktic 18 o records are pinned to the agemodel and averaged together to provide a record of glacial variability major deglaciation features are identified over the last 2 ma and a remarkable 33 out of 36 occur when earth s obliquity is anomalously large during the early pleistocene deglaciations occur nearly every obliquity cycle giving a 40 ka timescale while late pleistocene deglaciations more often skip one or two obliquity beats corresponding to 80 or 120 ka glacial cycles which on average give the 100 ka variability this continuous obliquity pacing indicates that the glacial theory can be simplified an explanation for the 100 ka glacial cycles only requires a change in the likelihood of skipping an obliquity cycle rather than new sources of long period variability furthermore changes in glacial variability are not marked by any single transition so much as they exhibit a steady progression over the entire pleistocene the mean variance skewness and timescale associated with the glacial cycles all exhibit an approximately linear trend over the last 2 ma a simple model having an obliquity modulated threshold and only three adjustable parameters is shown to reproduce the trends timing and spectral evolution associated with the pleistocene glacial variability

revised two feldspar geothermometry applied to sri lankan feldspars abstract two feldspar geothermometry is applied to granulite and amphibolite facies feldspars from sri lanka a procedure is suggested to correct for compositional changes due to retrograde resetting the feldspars derive from different geological units from different rocks involved in different acts of deformation and to feldspars which grew at different times in the temperature time deformation history of the parent rock string perthites from granulite facies rocks in the eastern highland complex hc which are the deepest in the pile yield highest temperatures of c 890 c 850 c feldspars that unmixed during or after f 4 give temperatures around 800 c 750 c in the south but 870 c 830 c in the north in the north near peak temperatures outlasted formation of the big late f 4 folds in the south temperature started to drop before f 4 temperatures obtained from mesoperthites of monzonitic to pegmatitic metamagmatic rocks from the west of the hc give 780 c 770 c interpreted as the minimum temperature of growth during f 4 and reflecting metamorphic equilibration a large metagranite in the nw vijayan nwv contains coarse string perthites yielding a temperature of metamorphic equilibration for phenocrysts and a temperature of metamorphic formation for coarse recrystallized grains of 880 c 840 c this proves that the nwv went through the same granulite facies deformation and metamorphism as the hc before it was swamped by strong migmatization at 751 c 710 c at a late stage whenever string perthites are deformed and recrystallize once or several times during falling temperature the original grains and first recrystallizates yield the highest the younger and younger generations of recrystallized grains successively lower temperatures of formation in the same sequence grains and plagioclase domains unmixed from them become successively less and finer thus there is a very good correlation between string perthite morphology and temperature of growth or equilibration preceding unmixing

coupled energy balance ice sheet model simulations of the glacial cycle a possible connection between terminations and terrigenous dust we apply a coupled energy balance ice sheet climate model in an investigation of northern hemisphere ice sheet advance and retreat over the last glacial cycle when driven only by orbital insolation variations the model predicts ice sheet advances over the continents of north america and eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses the orbital forcing alone however is unable to induce the observed rapid ice sheet retreat and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting in the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination these include marine instability thermohaline circulation effects carbon dioxide variations and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration for the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records of these various influences our analyses suggest that the albedo variations in the ice sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism using our parameterization of dirty snow in the ablation zone we find glacial retreat to be strongly accelerated such that complete collapse of the otherwise stable laurentide ice sheet ensues the last glacial maximum configurations of the laurentide and fennoscandian complexes are also brought into much closer accord with the ice 3g reconstruction of tushingham and peltier 1991 1992 and the ice 4g reconstruction of peltier 1994 when this effect is reasonably introduced

absolute chronology for major pleistocene advances of the laurentide ice sheet we used a 26 al 10 be burial isochron method to date the glacial stratigraphic section in missouri usa that records the largest advances of the laurentide ice sheet this permits an improved comparison of terrestrial and marine records of glaciation the first recorded advance of the laurentide ice sheet reached 39 n near the extreme southern limit of north american glaciation 2 4 ma the next advance to this latitude took place near the beginning of the mid pleistocene transition 1 3 ma and three more took place from 0 75 to 0 2 ma there is no evidence that the laurentide ice sheet advanced south of 45 47 n between 2 4 and 1 3 ma this chronology 1 shows that north american continental glaciation postdated cordilleran alpine glaciation 2 is consistent with the hypothesis that both of these events were threshold responses to tropical cooling 3 is consistent with the hypothesis that the first advance of the laurentide ice sheet was glaciologically anomalous due to the presence of deformable preglacial regolith 4 is not consistent with the hypothesis that this deformable regolith persisted until the mid pleistocene transition and 5 indicates that the increase in global ice volume at the mid pleistocene transition was at least in part the result of a more extensive laurentide ice sheet

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

chloride contamination in construction aggregates due to periodic saline water intrusion a case study in the kaluganga river estuary sri lanka the kaluganga river estuary is one of the main sources of construction sand in sri lanka salt water intrusion along this estuary due to extensive sand mining has increased over the years thus the focus of the current research is to understand the relationship between river sand mining salt water intrusion and the resultant effects on construction sand two surveys were conducted along the kaluganga estuary along an 11 km stretch from the river mouth at predetermined intervals to measure depth water quality profiles and to collect sediment samples these surveys were carried out during maximum spring tide first in a dry period and then in a wet period to understand hydrographic effects on the quality of river sands sand samples were analysed for absolute chloride content and grain size distribution results showed significant salt water intrusion during the dry period averaging 2 307 s cm 1 in surface waters throughout the surveyed 11 km stretch along with 3 818 s cm 1 average in bottom waters up to 5 6 km upstream from the river mouth causing above normal chloride content in the bottom sandy sediments the high chloride content in bottom sands was recorded up to 5 5 km from the river mouth making them unsuitable for construction purposes however during wet period salt water intrusion levels in the bottom waters were insignificant average 61 s cm 1 and the chloride content in bottom sediments was very low this study highlighted the requirement for regulations on river estuary sandmining for construction purposes

atlantic overturning responses to obliquity and precession over the last 3 myr this study analyzes 39 atlantic and seven pacific benthic 13c records to characterize obliquity and precession responses in atlantic overturning since 3 ma regional benthic 13c stacks are also analyzed a major transition in orbital responses is observed at 1 5 1 6 ma coincident with the first glacial shoaling of northern component water since 1 5 ma the phases of atlantic benthic 13c records from 2300 to 4000 m depth lag maximum obliquity by 59 6 7 kyr and june perihelion precession forcing by 133 8 5 kyr comparison with north atlantic sea surface temperature suggests that these orbital responses particularly precession in middle deep atlantic 13c are associated with changes in ocean heat transport and overturning rates the mid pleistocene transition had little effect on the obliquity and precession phases of benthic 13c but did result in a 50 decrease in the obliquity power of middle deep atlantic 13c at 0 6 ma

the missing glaciations of the middle pleistocene global glaciations have varied in size and magnitude since the early middle pleistocene transition 773 ka despite the apparent regular and high amplitude 100 ka pacing of glacial interglacial cycles recorded in marine isotope records the evidence on land indicates that patterns of glaciation varied dramatically between different glacial interglacial cycles for example marine isotope stages mis 8 10 and 14 are all noticeably absent from many terrestrial glacial records in north america and europe however globally the patterns are more complicated with major glaciations recorded in mis 8 in asia and in parts of the southern hemisphere such as patagonia for example this spatial variability in glaciation between glacial interglacial cycles is likely to be driven by ice volume changes in the west antarctic ice sheet and associated interhemispheric connections through ocean atmosphere circulatory changes the weak global glacial imprint in some glacial interglacial cycles is related to the pattern of global ice buildup this is caused by feedback mechanisms within glacial systems themselves that partly result from long term orbital changes driven by eccentricity

thermohaline circulation crisis and impacts during the mid pleistocene transition the mid pleistocene transition mpt marked a fundamental change in glacial interglacial periodicity when it increased from 41 thousand year to 100 thousand year cycles and developed higher amplitude climate variability without substantial changes in the milankovitch forcing here we document by using nd isotopes a major disruption of the ocean thermohaline circulation thc system during the mpt between marine isotope stages miss 25 and 21 at 950 to 860 thousand years ago which effectively marks the first 100 thousand year cycle including an exceptional weakening through a critical interglacial mis 23 at 900 thousand years ago its recovery into the post mpt 100 thousand year world is characterized by continued weak glacial thc the mpt ocean circulation crisis facilitated the coeval drawdown of atmospheric co2 and high latitude ice sheet growth generating the conditions that stabilized 100 thousand year cycles

synchronization of the climate system to eccentricity forcing and the 100 000 year problem the 100 000 year problem refers to an apparent mismatch between the strength of solar forcing associated with the 100 000 year cycle of eccentricity in the earth s orbit and the amplitude of glacial interglacial cycles numerical analyses suggest that recent glacial interglacial cycles can instead be explained by a phase locking between internal climate oscillations and the 413 000 year eccentricity cycle

north atlantic climate evolution through the plio pleistocene climate transitions during the plio pleistocene the earth witnessed the growth of large northern hemisphere ice sheets and profound changes in both north atlantic and global climate here we present a 3 2 myr long orbitally resolved alkenone sea surface temperature sst record from deep sea drilling project dsdp site 607 41 n 33 w water depth 3427 m in the north atlantic ocean we employ a multi proxy approach comparing these new observations with existing bottom water temperature bwt and stable isotope time series from the same site and sst time series from other sites shedding new light on plio pleistocene climate change north atlantic temperature records show a long term cooling with two major steps occurring during the late pliocene 3 1 to 2 4 ma and the mid pleistocene 1 5 to 0 8 ma closely timed with intervals of major change in northern hemisphere ice sheets existing evidence suggests that the late pliocene cooling may have been caused by a thresholded response to secular changes in atmospheric carbon dioxide co2 while an explanation for the mid pleistocene cooling may involve glacial interglacial changes in atmospheric co2 it seems to also require a change in the behavior of the ice sheets themselves north atlantic climate responses were closely phased with benthic oxygen isotope 18o changes during the 41 kyr world indicating a strong common northern hemisphere high latitude imprint on north atlantic climate signals after the mid pleistocene transition mpt north atlantic sst records and the site 607 benthic carbon isotope 13c record are more closely phased with 18o whereas bwt significantly leads 18o in the 100 kyr band suggesting a shift from a northern to a southern hemisphere influence on north atlantic bwt we propose that the expansion of the west antarctic ice sheet wais across the mpt increased the production and export of antarctic bottom water from the southern ocean and subsequently controlled its incursion into the north atlantic especially during glacial intervals it follows that the early 100 kyr response of bwt implies an early response of the wais relative to the northern hemisphere deglaciation thus in the 100 kyr world both northern hemisphere and southern hemisphere processes affect climate conditions in the north atlantic ocean

evolution of ocean temperature and ice volume through the mid pleistocene climate transition earth s climate underwent a fundamental change between 1250 and 700 thousand years ago the mid pleistocene transition mpt when the dominant periodicity of climate cycles changed from 41 thousand to 100 thousand years in the absence of substantial change in orbital forcing over this time an increase occurred in the amplitude of change of deep ocean foraminiferal oxygen isotopic ratios traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep ocean temperature we have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios our results suggest that the mpt was initiated by an abrupt increase in antarctic ice volume 900 thousand years ago we see no evidence of a pattern of gradual cooling but near freezing temperatures occur at every glacial maximum

improved chronostratigraphic reference curve of late neogene seawater 87sr 86sr we present a reference curve of seawater 87 sr 86 sr variation through the past 7 m y based on 455 samples of planktonic foraminifera from ocean drilling program odp site 758 in the indian ocean this single site curve is superior to extant curves because of several features 1 a continuous sedimentary section free of disturbances 2 a well determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale 3 a high temporal resolution sample interval averages 15 ka 4 an improved analytical reproducibility and 5 a uniformly high sample quality the gross structure of the site 758 curve is generally similar to that observed in previous work but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution at the 95 confidence level uncertainties on age estimates range from 0 60 m y in the lower pleistocene and upper miocene to 2 03 m y in the middle pliocene moreover this detailed record identifies two periods of steplike increases in seawater 87 sr 86 sr 6 10 to 5 80 ma and 1 46 to 1 13 ma that are characterized by rates of change of 150 10 6 m y these features if verified at other sites indicate abrupt changes in the sr mass balance of the ocean and may be used as precise event markers in late neogene marine sections

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

two phase change in co2 antarctic temperature and global climate during termination ii the end of the last glacial maximum termination i roughly 20 thousand years ago ka was marked by cooling in the northern hemisphere a weakening of the asian monsoon a rise in atmospheric co2 concentrations and warming over antarctica the sequence of events associated with the previous glacial interglacial transition termination ii roughly 136 ka is less well constrained here we present high resolution records of atmospheric co2 concentrations and isotopic composition of n2 an atmospheric temperature proxy from air bubbles in the epica dome c ice core that span termination ii we find that atmospheric co2 concentrations and antarctic temperature started increasing in phase around 136 ka but in a second phase of termination ii from 130 5 to 129 ka the rise in atmospheric co2 concentrations lagged that of antarctic temperature unequivocally we suggest that during this second phase the intensification of the low latitude hydrological cycle resulted in the development of a co2 sink which counteracted the co2 outgassing from the southern hemisphere oceans over this period

upb geochronology of the sri lankan basement abstract u pb ages of zircons and monazites from ortho and paragneisses representing the major crystalline basement units of sri lanka establish a new geochronological framework which is consistent with previously published work on sm nd model ages rb sr whole rock ages and u pb mineral ages in the highland complex three newly dated orthogneisses one previously dated by baur et al 1991 and one dated by kroner and williams 1992 all intruded 1 85 to 1 9 ga ago this widespread intrusion event occurred soon after the end of sediment deposition which is estimated at 1 9 to 2 0 ga using upper intercept ages of zircons from metapelites in the highland complex within the boundaries of this complex established by sm nd model age mapping milisenda et al 1994 only one orthogneiss has been found which is substantially younger than 1 8 ga this is the metatonalite from polonnaruwa which intruded 670 ma ago baur et al 1991 three orthogneisses from the vijayan complex which lies to the east of the highland complex yield similarly consistent crystallization ages of 1023 8 1016 28 and 1033 121 ma in the wanni complex which lies to the west of the highland complex a sillimanite bearing gneiss which is apparently of metasedimentary origin but contains zircons of igneous appearance yields a primary crystallization age of 793 4 ma consistent with a previously dated orthogneiss from kurunegala of 770 ma baur et al 1991 in addition single zircon data by kroner et al 1994 indicate primary crystallization ages of 1 0 to 1 1 ga for orthogneisses from the vavuniya charnockite province which is considered to be part of the wanni complex these results confirm the age contrast between the highland complex on the one hand and the vijayan complex and wanni complex on the other as previously inferred from sm nd model ages milisenda et al 1988 1994 post tectonic granitoids and pegmatites intruded throughout all three complexes 550 ma ago the age of regional high grade metamorphism mostly granulite facies in the highland complex mostly amphibolite facies in the vijayan complex and both granulite and amphibolite facies in the wanni complex is constrained to be less than 670 ma the primary crystallization age of the polonnaruwa metatonalite it is more closely defined by lower intercept zircon ages which include one concordant age of 608 4 ma and one discordant but well defined age of 611 23 ma and by a nearly concordant monazite age of 592 2 ma this indicates that peak metamorphic conditions occurred 610 ma ago numerous other lower intercept ages between 590 and 550 ma and post tectonic intrusion ages of 550 ma set a lower age limit for high grade conditions and deformation the temperature history between 610 and 550 ma is not yet well resolved and it is not clear whether there was a separate thermal event 550 ma ago previously published whole rock rb sr and sm nd isochrons for gneisses from the highland complex yielding ages of 2 ga and greater and previously interpreted to date the time of granulite grade metamorphism must be reinterpreted in the light of the new data presented here as well as other recently published results e g burton and o nions 1990b baur et al 1991 and in view of the accumulated evidence for very limited element mobility during high grade metamorphism these whole rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite facies metamorphism

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

the precambrian of sri lanka a historical review early studies of the geology of sri lanka lasted from the beginning of the century until 1935 the modern phase began in the forties whereas intensive studies of the precambrian rocks took place in the last ten years the term vijayan gneiss first used for the gneisses around colombo was changed in 1947 to vijayan series for all rocks outside the central khondalite charnockite belt the latter was named the highland series in 1961 and the vijayan series was subdivided into an eastern and a western component and the name was altered to vijayan complex in 1978 in 1959 a southwestern group was recognized and in 1935 the kadugannawa gneiss was named at present there is a general agreement that the following nomenclature should be used by all workers on the precambrian of sri lanka highland complex hc formerly highland series and southwestern group an intimate association of interbanded ortho and paragneisses including pelitic gneiss metaquartzite marble and charnockitic gneiss which forms the broad belt of rocks extending from northeast to southwest through the central highlands of the island outliers of hc rocks tectonic klippen are found in the vijayan complex at kataragama and maligawila vijayan complex vc formerly eastern vijayan complex consisting of migmatites granitic gneisses granitoids and scattered metasediments and occurring in the east and south of the hc wanni complex wc formerly western vijayan complex consisting of migmatites granitic gneisses charnockitic gneisses minor metasediments and granitoids lying west of the hc kadugannawa complex kc partly the former kadugannawa gneiss for the hornblende and biotite hornblende gneisses within the elongate synformal basins arenas around kandy in the early sixties it was noted that the highland series rocks had been subjected to granulite facies metamorphism and the vijayan series rocks to amphibolite facies metamorphism charnockitic rocks in the highland series showed the characteristics of metamorphic rocks recently near peak metamorphic conditions in the hc were established at 770 900 c and 5 kbar in the west wc to 9 kbar in the east hc with a pronounced regional gradient in palaeopressure relict inclusions of staurolite and kyanite in garnet of hc pelitic gneisses have been shown to document a late stage of a prograde metamorphic p t path a retrograde p t path with coronitic assemblages of retrograde mineral phases like andalusite has also been recorded outliers of hc rocks in the vc are klippen thrust over the vc parts of kc have been interpreted as a metamorphosed layered basic complex in the late seventies three phases of deformation were recognized in the hc presently four phases are known to occur an early view was that the highland series was younger than the vijayan series the former sediments having been laid down on a vijayan floor the hc is now known to be older than the vc the boundary between them being a thrust contact the exact position and nature of the hc wc boundary however is still to be resolved recent geochronological data suggest that 1 hc sedimentation as well as igneous activity in the form of basic lava flows basic sills and dykes took place 2 0 ga ago 2 intrusion of the earliest granitoids into the hc took place 1 8 to 1 94 ga ago 3 orthogneisses of the wc were intruded between 0 67 and 1 1 ga ago 4 orthogneisses of the vc were intruded 1030 to 1040 ma ago and 5 high grade regional metamorphism of the hc vc and wc took place between 455 and 610 ma ago

simulating the mid pleistocene transition through regolith removal abstract quaternary 18o ice volume proxy records show a transition from high frequency small amplitude glacial cycles to low frequency large amplitude glacial cycles this reorganization of the climate system termed the mid pleistocene transition mpt is thought to reflect a change in land ice response to orbital forcing despite no significant change in orbital cycles during this period one potential explanation for the mpt proposes that gradual erosion of high latitude northern hemisphere regolith by multiple cycles of glaciation caused a transition in ice sheet response to external forcing here we explore this regolith hypothesis using a complex earth system model we show that simulating a transition from deformable sediment to crystalline bedrock produces an evolution in ice volume response similar to proxy reconstructions of the mpt the simulated change in ice volume response is due to a combination of climate and ice flow changes with crystalline bedrock producing thicker colder ice sheets that accumulate more snowfall and have a smaller ablation zone further experiments that include transient eccentricity amplifying co2 forcing show only small differences in ice response compared to those with orbital forcing only suggesting that cycles of co2 were not the primary cause of the mpt

age of metamorphism in the high grade rocks of sri lanka controversy concerning the age of regional granulite grade metamorphism in sri lanka stems from the fact that different isotopic systems apparently record different events published isotopic data show that zircons from igneous and sedimentary protoliths experienced considerable pb loss at 550 ma and that new growth of zircon monazite rutile and garnet between 539 pm 6 and 608 pm 3 ma probably dates near peak metamorphic conditions upper concordia intercept ages between 670 and 1950 ma for magmatic zircons from high grade orthogneisses and charnockites date igneous emplacement and also support the occurrence of granulite metamorphism after 670 ma a granulite of igneous origin from the kataragama klippe in southern sri lanka yielded discordant ion microprobe data that suggest emplacement of the protolith at 1770 1880 ma thus a rb sr whole rock age of 1930 pm 130 ma from the same suite is unlikely to reflect isotopic homogenization during granulite formation and we interpret this and oth

on the mid pleistocene transition to 100 kyr glacial cycles and the asymmetry between glaciation and deglaciation times 1 a mechanism is proposed for the mid pleistocene transition from a dominant periodicity of 41 kyr to 100 kyr in glacial oscillations the same mechanism is shown to also explain the asymmetry between the long glaciation and short deglaciation phases of each cycle since that transition versus the symmetry of the 41 kyr oscillations prior to the transition these features arise naturally within the framework of the sea ice switch glacial cycle mechanism of gildor and tziperman 2000 as a result of the gradual cooling of the deep ocean during the pleistocene this cooling results in a change of the relation between atmospheric temperature and the rates of accumulation and ablation of continentals ice sheets it is this latter change that leads to the activation of the sea ice switch and therefore to the initiation of the 100 kyr oscillations the gradual glaciation and rapid deglaciations during these oscillations occur because the mean value of the ice sheet ablation is not far from the maximum rate of snow accumulation during warm periods this proximity of mean ablation and maximum accumulation rates is shown to be also a consequence of the mid pleistocene gradual cooling of the deep ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 1620 global change climate dynamics 3309 1640 global change remote sensing 4267 oceanography general paleoceanography keywords climate dynamics glacial cycles mid pleistocene transition 100 kyr 41 kyr

no glacial interglacial cycle in the ice volume simulated under a constant astronomical forcing and a variable co2 the relative impact of the co2 concentration on climate over the last 200 kyr is investigated here using the louvain la neuve 2 dimension northern hemisphere lln 2 d nh climate model forced by different constant insolations and the variable co2 concentration reconstructed from the vostok ice core record under such conditions the 100 kyr cycle in the northern hemisphere ice volume cannot be reproduced either a warm orbit prevents the ice sheets from forming or a present day and colder orbits do not allow them tol melt after being formed the global hemispheric temperature however continues to respond directly to the atmospheric co2 concentration changes

petrology of granulite facies metapelites of the highland complex sri lanka implications for the metamorphic zonation and the p t path in the pan african granulite facies highland complex of sri lanka the metamorphic grade increases from the central northwestern part towards boththe east and the southeast in metapelites the following prograde changes in metamorphic assemblage are recognized biotite ilmenite garnet or co cordierite sillimanite alkali feldspar quartz in the central northwestern part give way to garnet rutile sillimanite ilmenite or biotite alkali feldspar quartz in the e and se and are replaced by gar garnet cordierite biotite magnetite ilmenite alkali feldspar quartz bearing assemblages in the southwest cordierite coexisting with garnet is restricted to the western highland complex where metamorphic pressures of 5 7 kbar havebeen determined in the eastern highland complex where garnet clinopyroxene quartz is stable in metabasic rocks metamorphic pressures of 8 10 kbar have been obtained from barometers on metapelites phase relations of metapelites as well as geobarometry point to a continuous metamorphic transition between the eastern and the western part of the highland complex a clockwise p t path for the whole highland complex is indicated by the succession kyanite sillimanite andalusite an even earlier stage of the prograde metamorphism documented by sillimanite needles older than kyanite inclusions in garnet hhave been found in two samples of the eastern highland complex this points to a strong pressure increase at amphibolite facies conditions probably due to tectonic thickening of the crust during prograde metamorphism other inclusion assemblages in garnet and reaction textures indicate the breakdown of staurolite to kyanite spinel garnet and subsequent continued garnet growth in the sillimanite field at the expense of biotite sillimanite and quartz during further heating ternary feldspars in metapelites bear evidence of extreme metamorphic temperatures of about 900 c previously only indicated by coexisting pyroxenes in metabasic and charnockitic rocks within a few garnets inclusions of staurolite which are ti rich 1 34 wt tio2 and richer in fe xfe 0 745 than the enclosing garnet have survived the very high grade metamorphism the high grade stage of the prograde metamorphism was accompanied by strong flattening and a general n s stretching which is many metapelites ceased after peak temperatures but proceeded in others tectonic uplift of the whole highland complex occurred only after a phase of near isobaric cooling in the lower crust zircon 610 ma and biotite cooling ages 460 ma holzl et al 1991 as well as retrograde p t paths which passed the andalusite stability field are similar in the eastern and western part of the highland complex this suggests that the lower crustal cross section exposed in the highland complex was already tilted during cooling

sea level and deep sea temperature variability over the past 5 3 million years ice volume and hence sea level and deep sea temperature are key measures of global climate change sea level has been documented using several independent methods over the past 0 5 million years myr older periods however lack such independent validation all existing records are related to deep sea oxygen isotope 18o data that are influenced by processes unrelated to sea level for deep sea temperature only one continuous high resolution mg ca based record exists with related sea level estimates spanning the past 1 5 myr here we present a novel sea level reconstruction with associated estimates of deep sea temperature which independently validates the previous 0 1 5 myr reconstruction and extends it back to 5 3 myr ago we find that deep sea temperature and sea level generally decreased through time but distinctly out of synchrony which is remarkable given the importance of ice albedo feedbacks on the radiative forcing of climate in particular we observe a large temporal offset during the onset of plio pleistocene ice ages between a marked cooling step at 2 73 myr ago and the first major glaciation at 2 15 myr ago last we tentatively infer that ice sheets may have grown largest during glacials with more modest reductions in deep sea temperature a novel approach to the estimation of sea level and deep sea temperature has been used to determine these quantities over the past 5 3 million years this approach based on oxygen isotope records from the eastern mediterranean shows that temperature and sea level histories are broadly correlated but also show intriguing temporal offsets most records of past sea level variation and associated changes in ocean temperature rely on variations in oxygen isotopes recorded in deep sea marine microorganisms these signals unfortunately are confounded by other influences making it difficult to extract a clear climate related signal eelco rohling and colleagues now present a novel approach to estimating sea level and deep sea temperature variations for the past 5 3 million years the record based on oxygen isotope variations from the eastern mediterranean avoids the issues of the deep sea approach and generally corresponds to an independent record of the past 1 6 million years as expected temperature and sea level are broadly correlated but also show intriguing temporal discontinuities as at the initiation of plio pleistocene ice ages when a major sea level drop lagged a sharp temperature decline by several hundred thousand years

are the 41 kyr glacial oscillations a linear response to milankovitch forcing abstract the characteristics of glacial oscillations changed drastically 0 8 ma ago at the mid pleistocene transition during the past 0 8 ma the 100 kyr glacial interglacial oscillations were strongly asymmetric i e long glacial intervals of growth followed by rapid intervals of deglaciation the 40 kyr oscillations prior to the mid pleistocene transition were of a smaller amplitude and less asymmetrical looking the smaller amplitude apparently symmetric form and period that matches that of obliquity suggests that these oscillations were a linear response to milankovitch forcing while the 100 kyr oscillations are attributed either to some nonlinear self sustained variability due to a mechanism internal to the climate system itself or to nonlinear amplification of the insolation forcing the significant strengthening of the 100 kyr eccentricity power in the past 800 kyr is one of the intriguing questions of climate history here we show that glacial interglacial oscillations pre mid pleistocene transition are in fact significantly asymmetric this asymmetry may contradict a straight forward linear milankovitch explanation and we therefore suggest that the glacial oscillations before and after the transition may both be explained as self sustained variability although the possibility of nonlinear response to insolation forcing still exists the role of milankovitch forcing is in setting the phase of the oscillations e g time of terminations and their period rather in being the main driving force of the oscillations this is demonstrated using a simple model based on the sea ice switch mechanism of gildor and tziperman paleoceanography 15 2000 605

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

velocity profiles in steep open channel flows velocity measurements with a fiber optic laser doppler anemometer were conducted in steep open channel flows over smooth and imcompletely rough beds the channel slopes were changed from 1 500 and 1 25 by steps and as a result the froude number changed from 0 5 to 3 the law of the wall and the distributions of turbulence intensity were examined in more detail the von karman constant n in the log law was verified to be universally equal to 0 41 on the other hand the integral constant ia n in the log law coincided with the usual value of 5 29 in subcritical flows whereas it decreased with an increase of the bed slope in supercritical flows this decrease of ia n was explained by a decrease of the damping factor ib n in the van driest s mixing length formula the decrease of ib n was well correlated with an increase of the friction velocity and the roughness the streamwise turbulence intensity became smaller in the near wall region than the semiempirical universal formula as the friction velocity became larger

drag reduction in turbulent muddy seawater flows and some sedimentary consequences turbulent saline flows which contain low concentrations of suspended clay 10 g 1 1 have previously been reported to possess dramatically different boundary layer characteristics from clearwater flows and to exhibit the phenomenon of drag reduction drag reduction causes significantly lower friction factors and higher erosion thresholds in muddy saline flows than predicted by experimental data gained from fresh water flows using the law of the wall to estimate bed shear stress confirmation is given by experimental results on drag reduction using non intrusive laser doppler measurements in seawater flows with 2 2 g 1 1 maximum concentration of suspended clay increasing the concentration of clay caused progressively lower velocities near the wall due to a gradual thickening of the buffer region of the turbulent boundary layer several aspects of sediment transport deposition and bedform development in natural marine environments are thus considered to be significantly affected by the drag reduction process

geochemical constraints on the regolith hypothesis for the middle pleistocene transition the transition from 41 to 100 kyr glacial cycles and concomitant increase in global ice volume 1 ma remain an enigmatic feature of late cenozoic climate here we examine the petrology mineralogy and geochemistry of the silicate fraction of tills spanning the past 2 ma from the north central united states to evaluate the hypothesis that this so called middle pleistocene transition mpt occurred by erosion of regolith and subsequent exposure of underlying canadian shield bedrock by the laurentide ice sheet these data indicate that late pliocene tills are depleted in crystalline lithologies unstable minerals and major element oxides derived from plagioclase and ferromagnesians and are enriched in kaolinite quartz iron oxides tio2bearing resistates and meteoric 10 be in contrast early and middle pleistocene tills show enrichment in crystalline lithologies stable minerals and major oxides derived from plagioclase and ferromagnesians and depletion in meteoric 10 be whereas late pleistocene tills show major element concentrations that are most similar to that of fresh shield bedrock marine isotope records of sr os and hf show significant changes around the mpt that are consistent with the removal of a regolith and the exhumation of fresh silicate bedrock these results indicate that ice sheets initially expanded on highly weathered bedrock and progressively exhumed a fresher rock source thereby supporting the hypothesis that a change in the composition of the substrate underlying ice sheets best explains the origin of the mpt d 2004 elsevier b v all rights reserved

deformation of a high grade gondwana fragment sri lanka abstract the sri lankan gondwana fragment is a high grade gneiss terrain consisting of at least three crustal units namely the wanni complex wc the highland complex hc and the vijayan complex vc which were juxtaposed during the pan african collision at least six phases of ductile deformation were recognized in the smallest gondwana fragment sri lanka there is microscopic and mesoscopic evidence for a first deformation d 1 which occurred during prograde metamorphism but before the main granulite facies metamorphism this first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts microfolds in some metapelites and quartzo feldspathic gneisses and by a s 1 foliation preserved in crenulation folds and some rootless intrafolial folds d 2 has two main stages of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation l 2 the first stage of d 2 may have been responsible for the formation of a crenulation cleavage of s 1 as seen in garnet porphyroblasts in some metapelites the compositional layering s 2 in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage f 2 folds are mesoscopic isoclinal rootless and occur as intrafolial folds and their limbs are highly attenuated major part of the deformation during the latter stage of d 2 was non coaxial many minor and large scale recumbent isoclinal folds f 3 were produced during d 3 both d 2 and d 3 were coeval with the peak conditions of granulite facies metamorphism d 4 produced very large gentle nearly e w upright folds probably during the beginning of uplift of the entire high grade assemblage from deeper crustal levels still under granulite facies conditions d 5 is the second strongest deformation and was responsible for the large scale upright folds f 5 which control the present map pattern of sri lanka the superimposition of d 5 folds on d 4 folds gave rise to a large scale type i fold interference pattern of ramsay 1967 d 6 deformation caused local refolding of the f 5 folds and was accompanied by amphibolite facies conditions during uplift the deformation represented by the majority of vein type structures such as patchy charnockite veins cordierite bearing veins and graphite veins clearly post date d 5 it is argued that the wc the hc and the vc in sri lanka may be three distinctly different proterozoic crustal units brought into contact by tectonism in late proterozoic at two different stages

the first glacial maximum in north america despite marine evidence for at least 50 pliocene pleistocene ice sheet advances only the most recent one has been accurately reconstructed from terrestrial evidence because there are few techniques for dating older glacial deposits here we show that the cosmic ray produced nuclides beryllium 10 and aluminum 26 can be used to date tills that overlie paleosols

remote determination of the velocity index and mean streamwise velocity profiles when determining volumetric discharge from surface measurements of currents in a river or open channel the velocity index is typically used to convert surface velocities to depth averaged velocities the velocity index is given by k ub usurf where ub is the depth averaged velocity and usurf is the local surface velocity the usgs united states geological survey standard value for this coefficient k 0 85 was determined from a series of laboratory experiments and has been widely used in the field and in laboratory measurements of volumetric discharge despite evidence that the velocity index is site specific numerous studies have documented that the velocity index varies with reynolds number flow depth relative bed roughness and with the presence of secondary flows a remote method of determining depth averaged velocity and hence the velocity index is developed here the technique leverages the findings of johnson and cowen 2017 and permits remote determination of the velocity power law exponent thereby enabling remote prediction of the vertical structure of the mean streamwise velocity the depth averaged velocity and the velocity index

isotopic mapping of age provinces in precambrian high grade terrains sri lanka nd model ages of amphibolite and granulite grade rocks in sri lanka form a simple regional pattern that broadly correlates with mappable geological units and is in effect an isotopic map of the island s basement the granulite grade units of the highland group and southwest group have model ages of 2 2 3 0 ga indicating derivation mainly from late archean sources they are bounded to the east and west by late proterozoic gneisses of the vijayan complex with model ages of 1 1 2 0 ga the isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the vijayan gneisses are retrograde equivalents of the highland granulites sri lanka is not a direct continuation of the archean dharwar craton of southern india identification of vijayan type juvenile crustal terrains in other gondwana fragments may play a key role in determining the precise attachment of southern india sri lanka in eastern gondwana

stability of north atlantic water masses in face of pronounced climate variability during the pleistocene 1 geochemical profiles from the north atlantic ocean suggest that the vertical 13c structure of the water column at intermediate depths did not change significantly between glacial and interglacial time over much of the pleistocene despite large changes in ice volume and iceberg delivery from nearby landmasses the most anomalous 13c profiles are from the extreme interglaciations of the late pleistocene this compilation of data suggests that unlike today an extreme interglaciation the two primary sources of northern deep water norwegian greenland sea and labrador sea subpolar north atlantic had different characteristic 13c values over most of the pleistocene we speculate that the current open sea ice conditions in the norwegian greenland sea are a relatively rare occurrence and that the high 13c deep water that forms in this region today is geologically unusual if northern source deep waters can have highly variable 13c then this likelihood must be considered when inferring past circulation changes from benthic 13c records

modelling northern hemisphere ice volume over the last 3 ma the northern hemisphere ice sheet volumes over the last 3 ma were simulated using the lln 2 d model the forcings were both insolation and co2 concentration different atmospheric co2 scenarios were used because of a lack of co2 reconstruction over this remote past with constant co2 concentrations the simulated ice volume does not show any gradual increase as recorded in the marine sediments moreover its spectrum changes according to the chosen value of co2 when co2 is fixed at 220 ppmv the simulated ice volume is dominated by the similar to 100 ka period if a pre industrial co2 concentration of 280 ppmv is used the simulation is dominated by the similar to 41 ka period by using a linearly decreasing co2 concentration going from 320 ppmv at 3 ma bp to 200 ppmv at present the simulated changes in the power spectrum are in agreement with those obtained from the sedimentary records roughly before 1 ma bp the dominating period is similar to 41 ka afterwards the similar to 100 ka period becomes dominant this transition is accompanied by a gradual increase of the ice volume potential mechanisms for this transition are discussed using cyclic co2 fluctuations over the last 0 6 ma does not change significantly the spectral characteristics of the simulated ice volume but amplifies the amplitude of its variations c 1998 elsevier science ltd all rights reserved

african linkage of precambrian sri lanka new age and isotopic data show that the high grade basement rocks of sri lanka were not linked to the archaean granulite domain of southern india but experienced their main structural and metamorphic development during the pan african event some 950 to 550 ma ago this occurred when west gondwana and east gondwana collided to form one of the longest collisional structures in the supercontinent the mozambique belt that extends from mozambique to ethiopia and sudan a major tectonic boundary interpreted as a thrust zone divides the highland southwestern complex in the central part of sri lanka from the vijayan complex in the e and se the former is interpreted to represent the remnant of a once extensive passive margin extending west in a gondwana reconstruction via madagasgar to tanzania and mozambique the vijayan complex may have been part of a separate continental margin plutonic assemblage and its collision with the highland southwestern complex marks the final amalgamation of east and west gondwana into a supercontinent some 550 ma ago the sri lankan granulites cannot be correlated with the distinctly older granulites of the eastern ghats belt of india and this suggests that sri lanka was situated close to the se coast of madagascar in a gondwana reconstruction

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

the origin of the pink granites of sri lanka another view abstract pink granites are common throughout the precambrian metasedimentary terrain of sri lanka and play a significant role in reconstructing the geologic evolution of this country essentially microcline bearing rocks they form conformable layers with the associated metasedimentary rocks and are always interlayered and associated with rocks possessing amphibolite facies mineral assemblages this association is seen throughout the terrain at all scales further they always tend to fold around the major structures of the terrain and contain conformable layers and enclaves of rare metasediments and have a gneissic appearance in places their field relations associations mineralogy petrography petrofabrics chemistry and also age suggest that they are metasediments probably arkoses which have undergone the same set of metamorphic and deformational events as other precambrian metamorphic rocks of sri lanka their present granite like character does not necessarily imply that they are the products of crystallization of magmatic or anatectic melts or that they have had a replacement origin rather chemical composition has determined their present granite like character due to granitization by simple isochemical recrystallization treptomorphism during highgrade metamorphism at a ph2o which was sufficient to retard their grade to amphibolite facies under granulite facies metamorphic conditions at which the other metasediments of sri lanka have metamorphosed

plio pleistocene climate sensitivity evaluated using high resolution co2 records theory and climate modelling suggest that the sensitivity of earth s climate to changes in radiative forcing could depend on the background climate however palaeoclimate data have thus far been insufficient to provide a conclusive test of this prediction here we present atmospheric carbon dioxide co2 reconstructions based on multi site boron isotope records from the late pliocene epoch 3 3 to 2 3 million years ago we find that earth s climate sensitivity to co2 based radiative forcing earth system sensitivity was half as strong during the warm pliocene as during the cold late pleistocene epoch 0 8 to 0 01 million years ago we attribute this difference to the radiative impacts of continental ice volume changes the ice albedo feedback during the late pleistocene because equilibrium climate sensitivity is identical for the two intervals when we account for such impacts using sea level reconstructions we conclude that on a global scale no unexpected climate feedbacks operated during the warm pliocene and that predictions of equilibrium climate sensitivity excluding long term ice albedo feedbacks for our pliocene like future with co2 levels up to maximum pliocene levels of 450 parts per million are well described by the currently accepted range of an increase of 1 5 k to 4 5 k per doubling of co2

u pb isotopic systematics of zircons from prograde and retrograde transition zones in high grade orthogneisses sri lanka we present u pb zircon isotopic data from locally restricted prograde arrested in situ charnockitization and retrograde metamorphic transition zones which are well exposed in proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the highland group in sri lanka these granitoid rocks yield apparent ages of 1942 pm 22 ma 770 ma 660 ma and 560 ma all samples show severe pb loss some 550 560 ma ago the main phase of granulite formation could not be dated unambiguously but is bracketed between 660 ma and 550 ma the pervasive pb loss event around 550 560 ma reflects the end of this period of high grade metamorphism and was associated with widespread igneous activity and retrogression this is constrained by the 550 pm 3 ma intrusion age for a post tectonic granite we relate this late phase of thermal activity to crustal uplift of the sri lankan granulites our data unambiguously prove the high grade history of the sri lanka gneisses to be a late

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

atmospheric carbon dioxide concentration across the mid pleistocene transition the dominant period of pleistocene glacial cycles changed during the mid pleistocene from 40 000 years to 100 000 years for as yet unknown reasons here we present a 2 1 million year record of sea surface partial pressure of co2 pco2 based on boron isotopes in planktic foraminifer shells which suggests that the atmospheric partial pressure of co2 pco2 was relatively stable before the mid pleistocene climate transition glacial pco2 was approximately 31 microatmospheres higher before the transition more than 1 million years ago but interglacial pco2 was similar to that of late pleistocene interglacial cycles 450 000 years ago these estimates are consistent with a close linkage between atmospheric co2 concentration and global climate but the lack of a gradual decrease in interglacial pco2 does not support the suggestion that a long term drawdown of atmospheric co2 was the main cause of the climate transition