interhemispheric climate links revealed by a late glacial cooling episode in southern chile understanding the relative timings of climate events in the northern and southern hemispheres is a prerequisite for determining the causes of abrupt climate changes but climate records from the patagonian andes1 2 3 4 and new zealand5 6 7 8 for the period of transition from glacial to interglacial conditions about 14 6 10 kyr before present as determined by radiocarbon dating show varying degrees of correlation with similar records from the northern hemisphere it is necessary to resolve these apparent discrepancies in order to be able to assess the relative roles of northern hemisphere ice sheets and oceanic atmospheric and astronomical influences in initiating climate change in the late glacial period here we report pollen records from three sites in the lake district of southern chile 41 s from which we infer conditions similar to modern climate between about 13 and 12 2 14c kyr before present bp followed by cooling events at about 12 2 and 11 4 14c kyr bp and then by a warming at about 9 8 14c kyr bp these events were nearly synchronous with important palaeoclimate changes recorded in the north atlantic region9 supporting the idea that interhemispheric linkage through the atmosphere was the primary control on climate during the last deglaciation in other regions of the southern hemisphere where climate events are not in phase with those in the northern hemisphere local oceanic influences may have counteracted the effects that propagated through the atmosphere

interhemispheric synchrony of late glacial climatic instability as recorded in proglacial lake mascardi argentina several high resolution continental records have been reported recently in sites in south america but the extent to which climatic variations were synchronous between the northern and southern hemispheres during the late glacial holocene transition and the causes of the climatic changes remain open questions previous investigations indicated that east of the andes the middle and high latitudes of south america warmed uniformly and rapidly from 13 000 14c yr bp with no indication of subsequent climate fluctuations equivalent for example to the younger dryas cooling here we present a multiproxy continuous record radiocarbon dated by accelerated mass spectroscopy from proglacial lake mascardi in argentina the results show that unstable climatic conditions comparable to those described from records obtained in the northern hemisphere dominated the late glacial holocene transition in argentina at this latitude furthermore a significant advance of the tronador ice cap which feeds lake mascardi occurred during the younger dryas chronozone this instability suggests a step wise climatic history reflecting a global rather than regional forcing mechanism the lake mascardi record therefore provides strong support for the hypothesis that ocean atmosphere interaction rather than global ocean circulation alone governed interhemispheric climate teleconnections during the last deglaciation 1997 john wiley sons ltd

petrography and reservoir physics ii relating thin section porosity to capillary pressure the association between pore types and throat size porosity in reservoir rocks is configured into a few types of pores whose size and shape are controlled by depositional fabric and postdepositional processes the size shape and abundance of each pore type can be objectively determined from thin section using image analysis and pattern recognition procedures each pore type tends to be associated with a limited range of throat sizes the association between pore type and throat size can be determined using regression procedures linking pore type data obtained from thin section with capillary pressure data to do so a set of samples is required wherein the association between pore type and throat size is fixed but where pore type proportions vary between samples this condition is met by a sample suite representing res rvoir facies from a single core or in many cases from a single field the relationship between pore type and throat size is an effective means to relate reservoirs in terms of the efficiency of the porous microstructure to multiphase flow parameters derived from the relationship can be used to construct accurate physical models that subdivide physical response in terms of the contributions of each pore type

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

the evolution of the patagonian ice sheet from 35 ka to the present day patice abstract we present patice a gis database of patagonian glacial geomorphology and recalibrated chronological data patice includes 58 823 landforms and 1 669 geochronological ages and extends from 38 s to 55 s in southern south america we use these data to generate new empirical reconstructions of the patagonian ice sheet pis and subsequent ice masses and ice dammed palaeolakes at 35 ka 30 ka 25 ka 20 ka 15 ka 13 ka synchronous with the antarctic cold reversal 10 ka 5 ka 0 2 ka and 2011 ad at 35 ka the pis covered of 492 6 x103 km2 had a sea level equivalent of 1 496 mm was 350 km wide and 2090 km long and was grounded on the pacific continental shelf edge outlet glacier lobes remained topographically confined and the largest generated the suites of subglacial streamlined bedforms characteristic of ice streams the pis reached its maximum extent by 33 28 ka from 38 s to 48 s and earlier around 47 ka from 48 s southwards net retreat from maximum positions began by 25 ka with ice marginal stabilisation then at 21 18 ka which was then followed by rapid irreversible deglaciation by 15 ka the pis had separated into disparate ice masses draining into large ice dammed lakes along the eastern margin which strongly influenced rates of recession glacial readvances or stabilisations occurred at least at 14 13 ka 11 ka 6 5 ka 2 1 ka and 0 5 0 2 ka we suggest that 20th century glacial recession a 1 is occurring faster than at any time documented during the holocene

a record from lake cardiel of climate change in southern south america southern south america the only continental land mass lying between 38 s and the antarctic circle presents a rare opportunity to reconstruct terrestrial records of palaeoclimate in an area that is subject to shifts of polar and mid latitude wind and pressure belts in the bulk of this region patagonia are many hydrographically closed lakes which despite the palaeoclimatic significance of the region and the proven utility of lake level records in reconstructing past climates1 2 have escaped detailed study using sedimentary and geomorphic evidence together with 14c dating we present here a detailed lake level record for lake cardiel fig 1 the highest level of the past 21 000 years occurred at about 9 780 bp reminiscent of tropical lakes rather than those in northern mid latitudes1 2 the lake cardiel record corroborates and clarifies the timing of the wetness record inferred from pollen studies in patagonia

magnetostratigraphy of the miocene chiang muan formation northern thailand implication for revised chronology of the earliest miocene hominoid in southeast asia abstract a paleomagnetic study has been conducted on the miocene chiang muan formation in northern thailand in order to provide a chronology for the earliest large bodied miocene hominoid in southeast asia the chiang muan formation is mainly composed of clay silt and sand beds indicating lacustrine and fluvial environments paleomagnetic samples were collected from 124 horizons along an approximately 150 m thick section from the chiang muan formation at the opencast chiang muan mine rock magnetic experiments and stepwise thermal demagnetizations revealed that the main carrier of the magnetization of the sediments is magnetite and the samples have stable magnetization exceptionally stable magnetization is also carried by hematite which is represented by red colored sediments characteristic remanent magnetization chrm directions calculated by the principal component analysis revealed normal or reversed polarities of magnetization which allow the application of the reversal test of mcfadden and mcelhinny mcfadden p l mcelhinny m w 1990 classification of the reverse test in paleomagnetism geophys int 103 725 729 the mean paleomagnetic directions of the normal and reversed polarities passed the reversal test with a classification c indicating that the chiang muan formation preserved the primary magnetization in total five normal and four reversed polarity zones are recognized from the studied section based on paleontological age constraints this magnetostratigraphic column of the chiang muan formation correlates best with chron c5aan c5n of the geomagnetic polarity time scale gpts from the geological time scale gts2004 developed by gradstein et al gradstein f ogg j smith a eds 2004 a geological time scale 2004 cambridge univ press cambridge uk p 589 this correlation revealed that sedimentation of the chiang muan formation began approximately at 13 ma and continued until 9 8 ma with a mean sedimentation rate of approximately 4 2 cm ky the age of the earliest southeast asian hominoid is between 12 4 and 13 0 ma

full glacial late glacial palaeoclimate of the southern andes evidence from pollen beetle and glacial records palaeoecological studies carried out in the chilean lake district and chilotan archipelago 41 43 s record full glacial and late glacial pollen assemblages beginning just after 21000 and beetle assemblages after 18000 both sets extending until 10000 14c yr bp pollen records indicate that subantarctic parkland the vegetation of the early millennia of record changed after about 14000 yr bp to become open woodland and later north patagonian evergreen forest assemblages of plants and beetles responding more or less in unison to a strong rise in temperature 6 c behaved in accord at around 14000 until 13000 12500 yr bp the beetle fauna displaying a marked increase in obligate forest types during full glacial conditions 17400 16100 and 15300 and 14400 yr bp and in the late glacial interval after about 13000 yr bp however climate evidently coerced populations dissimilarly the pollen sequence showing an increase in plant taxa indicative of colder climate whereas the beetle fauna underwent little or no variation contrasting climate modes implied by plants and beetles may be attributed to differential responses to apparent low order temperature changes 2 3 c

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

paleomagnetism of large igneous provinces case study from west greenland north atlantic igneous province abstract we present new paleomagnetic and multi model stereo photogrammetry data from lava sequences in the west greenland part of the north atlantic igneous province naip the joint analyses of paleomagnetic and photogrammetric data yield a well defined paleomagnetic pole located at lat 73 6 n long 160 5 e n 44 95 6 2 k 13 1 age 61 55 ma which is statistically indistinguishable from a pole recently obtained for the eurasian part of the naip on faroe islands riisager et al earth planet sci lett 201 2002 261 276 combining the two datasets we obtain a joint naip paleomagnetic pole in greenland coordinates lat 71 1 n long 161 1 e n 87 95 4 3 k 13 6 age 61 54 ma the results presented here represent the first study in which photogrammetry profiles were photographed at the exact same locations where paleomagnetic fieldwork was carried out and a direct flow to flow comparison of the two datasets is possible photogrammetry is shown to be particularly useful because of i highly precise dip strike measurements and ii detailed field observations that can be made in the laboratory highly precise determination of the structural attitude of well exposed kanisut mb lava sequences demonstrates that their apparently reliable in field dip strike measurements typically are up to 6 wrong erroneous dip strike readings are particularly problematic as they offset paleomagnetic poles without affecting their confidence limits perhaps more important for large igneous provinces is the recognition of a variable temporal relationship between consecutive lava flows we demonstrate how correct interpretation of paleosecular variation facilitated by the detailed photogrammetry analysis is crucial for the rapidly emplaced vaigat formation lavas inaccurate tectonic correction non averaged paleosecular variation and unrecognized excursional directions may perhaps explain why coeval paleomagnetic poles from large igneous provinces are often discordant the difference between the joint naip paleomagnetitc pole and apparent polar wander path poles suggests that they may be less reliable than suggested by their confidence limits

landform analysis of the pueyrredon lake area in northwestern santa cruz argentina abstract the aim of this research was to analyze the landforms around pueyrredon lake nw santa cruz in argentina to contribute to the understanding of the paleoenvironmental evolution of patagonia during the late pleistocene early holocene the pueyrredon lake area belongs to the patagonian shield northwest of the santa cruz province 47 22 28 s 71 55 34 w characteristic landforms were identified through satellite images landsat 7 etm and field work these landforms include glacial troughs glacier striations and groove casts three fan deltas on the south margin of pueyrredon lake lacustrine and glacio lacustrine sediment and till deposits in addition short detailed stratigraphic logs were measured for paleoenvironmental analysis and several sedimentary deposits were sampled to characterize grain size variations all of the information is displayed on a geomorphological map the modern and ancient landforms were also analyzed based on their topographic characteristics which were determined by a digital elevation model dem geomorphological conditions of the erosional and depositional processes in the paleoglacial environment were analyzed using longitudinal and cross sections the results shed light on the climatic variability during the most recent geological period major changes in the landscape morphology were associated with glacier evolution and the development and division of the great lakes in the eastern andes in central patagonia

magnetic polarity stratigraphy stochastic properties of data sampling problems and the evaluation of interpretations a statistical model has been derived which can be used either to test the interpretation of a given magnetic stratigraphy or to make time estimates from the paleomagnetic data the model is based on the presently accepted statistical properties of the earth s magnetic field and the stochastic nature of the paleomagnetic sampling process given knowledge of the number of paleomagnetic sites the distribution of the sites an estimate of the geologic time span sampled and the mean length of the magnetic polarity zones the expected success of the sampling program can be calculated and compared with its actual performance in a number of test cases the theoretical results have agreed with realized results the mean time span for polarity intervals in the late neogene has been established from existing data at 120 000 years once the mean time span for polarity intervals has been established estimates for stratigraphic time span can be made by means of the statistical model of three models studied a time uniform sampling program is the most efficient for establishing a local magnetic polarity stratigraphy the least efficient strategy is an exponential distribution of samples

stability of the earth with respect to the spin axis for the last 130 million years abstract recently continental paleomagnetic data rotated into a hotspot reference frame have been used to suggest the earth rotated 18 with respect to the spin axis at 110 ma we test this true polar wander tpw hypothesis using paleomagnetic data from widespread granitic rocks of cratonal north america these data conflict with tpw predictions leading to rejection of the hypothesis the calculated polar displacement is instead consistent with motion of the atlantic hotspots during the mid cretaceous this analysis suggests that the time averaged position of the spin axis has deviated by no more than 5 over the last 130 million yr indicating that mantle mass heterogeneities have not changed rapidly enough to drive tpw

paleogeographic reconstruction of the tar san martin lacustrine system during late pleistocene to early holocene landscape availability and hunter gatherer circulation santa cruz argentina abstract this paper presents the paleogeographic reconstructions of flooded areas of the tar san martin lacustrine system during the late pleistocene holocene the dimensions of lacustrine and glaciolacustrine levels of this basin are higher than the current levels of the lakes the organic matter of lake levels recorded on stratigraphic profiles was described sampled and dated using ams while other paleobathymetric indicators were also identified height of the fan delta apex and paleo coast levels deriving out of the reconstruction of flooded areas it was established that the lake system was formed before 28 ka ago and would have reached its maximum expansion around 12 ka ago the first hunter gatherer occupation of the basin 11 ka corresponds to the period of initial contraction of the lake system and the composition of the current configuration of the coastlines this scenario would have allowed the exploration of western areas and the gradual incorporation of the forest to the action range of hunter gatherer populations similarly the levels of lakes tar and san martin would not have hindered the movement of human populations through the steppe throughout the holocene

chronology of bathymetric variations of the pueyrredon posadas salitroso lacustrine system during the late pleistocene to early holocene abstract the pueyrredon posadas salitroso lacustrine system pps ls is located in the andean patagonia region of argentina 47 22 28 s 71 55 34 w and represents a series of lakes formed as glaciers retreated during the late pleistocene and early holocene these lakes began as proglacial lakes and later evolved into periglacial lakes this research presents the chronological evolution of the pps ls bathymetric variations based on the analysis of stratigraphic profiles surveyed at different locations and the dates obtained from lake sediments four paleogeographic sketches representing the chronological evolution of the pps ls between 23 580 bp and 5208 bp have been developed based on the integration of these data

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

palaeoclimates in central and south america since 18 000 bp based on pollen and lake level records abstract the last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from central and south america a total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis 8 records from tropical and subtropical lowlands in central and south america 7 records from the central american highlands 26 records from the high andes north and south of the equator 2 records from the southern subtropics and 18 from the southern andes and subantarctic islands palaeoclimatic interpretation of the pollen records is semiquantitative based on bioclimatic parameters such as e g elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation additional palaeoclimatic information is obtained from lake level records derived either from dated shorelines or from the limnological history of lake sediment sections large scale palaeoclimatic comparison of the proxy data records indicates that full and late glacial climates throughout central and south america were cooler than today by about 4 to 5 c but that moisture patterns showed regional differentiation thus in both the southern subtropics and high latitudes in southern chile conditions were substantially moister than today during full and late glacial times whereas everywhere else conditions were far drier than today in terms of palaeocirculation this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation a scenario that has been suggested by palaeoclimatic simulations from general circulation models gcm another major palaeoclimatic aspect predicted by the gcm simulations is the suppressed early holocene summer monsoon in the southern subtropics although proxy records are few from this zone their palaeoenvironmental history during that time shows a period of aridity that would support the model one of the most controversial aspects of south american palaeoclimates concerns the existence of a younger dryas equivalent a cold period between 11 and 10 ka bp based on the existing data from south america that show a great diversity of palaeoclimatic trends the concept of an interval that is characterized by the same climatic signal as documented for the north atlantic seems too simplistic moreover at this point only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question the principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present and the realization of regions and periods where much more and concerted effort is needed to address problems of intra and interhemispheric palaeoclimatic correlations only then can questions concerning the mechanism of climatic change be tackled successfully

plumes in a convecting mantle models and observations for individual hotspots the motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented a new list of 44 possible hotspots with associated tracks has been compiled for all of them calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow plume source depth has usually been assumed to be the top of d but an alternative source depth at the 670 km discontinuity has also been considered using models of relative plate motions and boundaries hotspot tracks on plates have been calculated and compared with age data ocean floor topography and distribution of volcanics on continents absolute plate motions have been redetermined under consideration of hotspot motion using a new least squares method for the hawaiian and yellowstone hotspots source locations and hotspot motion have been computed for a total of up to 23 different models the results show plume conduits being tilted with source regions at the d moving in the lowermost mantle flow generally toward large scale upwellings under southern africa and the south central pacific hotspot surface motion often represents the horizontal component of midmantle flow which is frequently opposite to plate motion toward ridges and away from subduction zones in particular almost all models tested predict southward motion of the hawaii and kerguelen hotspots and westward motion of the iceland hotspot for models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as or better than for the fixed hotspot model but sometimes fixed hotspots give the best fit in some cases where the track ends at a subduction zone e g for the bowie hotspot results can give indications about the otherwise unknown age of the hotspot in other cases especially for the tahiti hotspot results suggest an origin shallower than d and in yet other cases particularly east africa the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes

relative hotspot motions versus true polar wander abstract the fixity of hotspots and mantle plume locations has long been axiomatic if the assumption of fixed hotspots is granted absolute plate motions and movements of the spin axis with respect to the hotspot framework defined by some as true polar wander tpw can be determined however this assumption can be tested by paleomagnetic data and such tests are gradually raising some doubts about the fixity of hotspots the result is that discrepancies between cretaceous and tertiary hotspot and paleomagnetic reference frames are now beginning to be interpreted as the result of plume drift within a convective mantle in the indo atlantic hotspots have remained relatively stationary with respect to the spin axis for the last 95 million yr however the pacific hotspots notably hawaii appear to have undergone large scale southward drift with respect to the spin axis during the early tertiary global paleomagnetic data do not indicate that any tpw occurred during the late cretaceous or tertiary although the early cretaceous paleomagnetic and hotspot frames for the indo atlantic realm can be interpreted as slow tpw direct estimates of paleolatitude and hotspot motion in particular the kerguelen hotspot challenge tpw as a global phenomenon at present we consider that the large early cretaceous discrepancy between hotspot and paleomagnetic data is best explained by southward drift of the atlantic hotspots prior to 95 ma

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

porosity prediction prior to drilling in sandstones of the kekiktuk formation mississippian north slope of alaska porosity in sandstones of the kekiktuk formation was successfully estimated prior to drilling of the 1 leffingwell wildcat well north slope of alaska the estimate was based on a calibration dataset used to evaluate the effects of 1 framework grain composition 2 depositional facies and 3 postdepositional processes on porosity of kekiktuk sandstones the sandstones of the kekiktuk formation are chert bearing sublitharenites and quartzarenites characterized by a homogeneous composition of the detrital framework in the study area thus mineral composition is not a major factor responsible for differences in reservoir quality based on outcrop and available core observations the kekiktuk formation was interpreted to include several wet fan deltas the depositional model suggested that the 1 leffingwell well would penetrate the distal fine grained facies of one such system a petrographic study indicated that in fine and very fine grained kekiktuk sandstones such as those predicted in the wildcat porosity was reduced primarily by silica cementation silica cementation in turn is related to burial history because of the relationship among porosity silica cementation and burial history burial history diagrams provided a measure of the effect of burial history on porosity in available calibration wells a synthetic burial history curve was constructed prior to drilling of the 1 leffingwell well from available seismic data this burial history curve was then used to estimate the well s porosity based on the previously established porosity burial history relationship

magnetostratigraphy and high resolution lithostratigraphy of the permian triassic boundary interval in central germany abstract a comprehensive lithostratigraphic and magnetostratigraphic study was carried out in order to provide a magnetic polarity scale for the lower buntsandstein of the classic germanic trias in its type area of central germany the 300 m thick lower buntsandstein consists of 20 small scale fining upward cycles they can be correlated over large parts of the central european basin using a combination of high resolution cyclic stratigraphy and gamma ray logging on the basis of this robust lithostratigraphic framework a very precise positioning and correlation of paleomagnetic results has been realized eight outcrops and one completely cored well were collected yielding a total of 900 paleomagnetic samples from about 80 of them a characteristic remanence was obtained the consistent correlation of lithostratigraphic units and magnetozones supports an early acquisition of the characteristic magnetizations in both the magnetite bearing gray lithologies as well as the hematite bearing red brown lithologies the inter section correlation of all investigated profiles allows the creation of a well defined composite magnetic polarity scale the uppermost zechstein comprises one normal znz and two reversed zry zrz magnetozones in the overlying lower buntsandstein are delineated three normal sn1 to sn3 and three reversed sr1 to sr3 well documented magnetozones which can be correlated with magnetic records from the boreal and tethyan realms the combination of magnetostratigraphic and biostratigraphic data suggests the base of the oolite alpha 2 within the beds of the so called graubankbereich lower buntsandstein as the position of the hindeodus parvus calibrated permian triassic boundary within the germanic trias it is situated within the lowermost part of a remarkable thick normal magnetozone sn1 which is a distinctive feature occurring in virtually all magnetic records across the permian triassic boundary

last glacial maximum environments in northwestern patagonia revealed by fossil small mammals abstract comparisons of historical and modern assemblages of mammals can yield important insights into patterns and processes of environmental change here we present the first analyses of small mammal assemblages present in northern patagonia during the last glacial maximum lgm using remains obtained from owl pellets excavated from an archeological cave site arroyo corral i levels vii v carbon dates of 22 400 21 530 cal yr bp we generate estimates of the minimum number of individuals for all species detected these estimates in turn are used to determine relative species abundances comparisons of these data with similar analyses of small mammal remains obtained from a second archeological site acoii levels iv v carbon dates of 10 010 9220 cal yr bp as well as from modern owl pellets reveal pronounced changes in relative species abundance since the lgm in particular euneomys chinchilloides and ctenomys sociabilis the predominant species during the lgm declined markedly suggesting a change from open bare habitat punctuated by patches of wet meadows and shrubs to the more densely vegetated mosaic of ecotone habitats found in this region today these data provide important new insights into the environmental changes that have occurred in northern patagonia over the last 20 000 years

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

empirical prediction of porosity and permeability in sandstones current efforts to predict porosity and permeability in sandstones prior to drilling are focused on empirical and process oriented models empirical predictions are based on the correlation between porosity and permeability and a limited number of parameters obtained from calibration data sets or estimated from appropriate geologic models of these parameters the most important are detrital composition grain size sorting and temperature history or pressure history or both despite its limitations the empirical approach provides accurate predrill predictions of reservoir quality in many sandstones containing not only primary but also secondary porosity and permeability predictions of the average porosity and permeability of target sandstones are arbitrarily defined h re as accurate if they fall within 2 porosity of the mean measured porosity and within the same order of magnitude as the mean measured permeability in a sample population representing the interval of interest the effectiveness of the empirical predictive approach is illustrated by case studies from the taranaki basin new zealand and yacheng field people s republic of china these studies indicate that empirical predictions are basin specific or even play specific and require at least some understanding of fundamental processes affecting reservoir quality of a given sandstone target process oriented approaches attempting to model the effect of diagenesis on reservoir quality are hampered by inadequate quantitative understanding of the processes responsible for preserving primary porosity and generating secondary porosity and permeability until adequate quantification of the sandstone diagenesis processes is achieved empirical models have a distinct advantage over process oriented models in providing reliable predictions of reservoir quality in many sandstone intervals

paleobiogeography of africa how distinct from gondwana and laurasia abstract although africa was south of the tethys sea and originally belonged to the gondwana its paleobiogeographical history appears to have been distinct from those of both gondwana and laurasia as early as the earliest cretaceous perhaps the late jurassic this history has been more complex than the classical one reconstructed in the context of a dual world gondwana vs laurasia geological and paleobiogeographical data show that africa was isolated from the mid cretaceous albian aptian to early miocene i e for ca 75 million years the isolation of africa was broken intermittently by discontinuous filter routes that linked it to some other gondwanan continents madagascar south america and perhaps india but mainly to laurasia interchanges with gondwana were rare and mainly out of africa dispersals whereas interchanges with laurasia were numerous and bidirectional although mainly from laurasia to africa despite these intermittent connections isolation resulted in remarkable absences poor diversity and emergence of endemic taxa in africa mammals suggest that an african faunal province might have appeared by late jurassic or earliest cretaceous times i e before the opening of the south atlantic during isolation africa was inhabited by vicariant west gondwanan taxa i e taxa inherited from the former south american african block that represent the african autochthonous forms and by immigrants that entered africa owing to filter routes nearly all or all immigrants were of laurasian origin trans tethyan dispersals between africa and laurasia were relatively frequent during the cretaceous and paleogene and are documented as early as the earliest cretaceous or perhaps late jurassic i e perhaps by the time of completion of the tethys between gondwana and laurasia they were permitted by the mediterranean tethyan sill a discontinuous route that connected africa to laurasia and was controlled by sea level changes interchanges first took place between southwestern europe and africa but by the middle eocene a second eastern route the iranian route involved southeastern europe and southwestern asia the iranian route was apparently the filtering precursor of the definitive connection between africa and eurasia the relationships and successive immigrations of mammal mostly placental clades in africa allow the recognition of five to seven phases of trans tethyan dispersals between africa and laurasia that range from the late cretaceous to the eocene oligocene transition these dispersal phases involve dispersals toward laurasia and or toward africa immigrations the immigrations in africa gave rise to faunal assemblages the african faunal strata afss all successful and typical african radiations have arisen from these afss we recognize four to six afss each characterized by a faunal association even major old african clades such as paenungulata or the still controversial afrotheria which belong to the oldest known afs involving placentals ultimately originated from a laurasian stem group africa was an important center of origin of various placental clades their success in africa is probably related to peculiar african conditions endemicity weak competition although strongly marked by endemicity the african placental fauna did not suffer extinctions of major clades when africa contacted eurasia the present geographic configuration began to take shape as early as the mid cretaceous at that time the last connections between africa and other gondwanan continents began to disappear whereas africa was already connected to eurasia by a comparatively effective route of interchange

plate motions in the north pacific the 43 ma nonevent the hawaiian emperor seamount chain in the north pacific ocean is commonly considered to have been produced by motion of the pacific plate over a hotspot if the hotspot is assumed to have remained fixed in a mantle frame of reference the 60 change in trend between the hawaiian and emperor portions of the chain results from a change in motion direction of the pacific plate relative to the mantle at 43 ma during middle eocene time such a large and abrupt direction change should result in significant plate reorganizations and tectonic events in the pacific and surrounding plates but no evidence for these events can be found in addition in this paper it is shown that there were no changes in relative plate motions between pacific area plates that can be associated with the hawaiian emperor bend this is done by calculating motion of the pacific farallon and kula plates relative to asia and north america using the plate circuit farallon pacific antarctica africa north america asia with no major regional tectonic event at the time of the bend it is suggested that there was no plate reorganization at 43 ma and that the emperor portion of the seamount chain was formed by a nonstationary hotspot

quaternary glaciations of the southern andes abstract the southern andes comprise the southernmost portion of the andean cordillera beginning at the edge of the puna altiplano lat 27 s and ending at isla de los estados lat 55 s the late cainozoic glacial record of these mountains spans the interval from the late miocene to the present and is one of the most complete to be found anywhere in the world this has arisen for several reasons i the conterminous mountain ice cap extended to the piedmont zone on both flanks of the range where the sedimentary and morphological record has been well preserved ii periodic volcanism mainly from monogenetic fissure eruptions of basalt east of the range and from central tephra producing cones along the mountain crest has provided opportunity for the preservation and radiometric dating of interbedded glacial deposits iii a tectonically induced interval of stream incision in the mid pleistocene and simultaneous uplift has preserved glacial sediments on interfluves iv in the chilean lakes region west of the mountains late quaternary glaciers terminated in a well vegetated landscape thus creating scope for radiocarbon dating of interbedded and incorporated organic materials consequently the last glaciation in the llanquihue area of chile is one of the best dated sequences in south america thus the llanquihue glaciation is proposed as the south american equivalent of the wisconsin and weichsel glaciations of north america and north west europe respectively

evidence for late paleozoic and mesozoic non dipole fields provides an explanation for the pangea reconstruction problems abstract paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole but in recent years it has become a concern that this assumption is not always valid because of long term contributions from non dipole fields in particular octupole fields we tested for the presence of a zonal octupole field in the late carboniferous permian mesozoic and early tertiary by comparing the observed paleomagnetic paleolatitude distributions for the laurentian north america and greenland and european landmasses with those predicted from the mean paleopoles such a test is possible for a good sized continent not subsequently deformed when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes if the field were purely dipolar coeval results should form a coherent dataset such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1 if the slopes of regression lines are systematically different from 1 0 for successive time intervals and granted that the earth s radius stayed the same then it is likely that non dipole fields can be held responsible regression line slopes standard errors for three intervals analyzed are 0 78 0 04 200 300 ma 0 93 0 11 120 200 ma and 0 82 0 06 40 120 ma as might be expected with a non dipole contribution to the total geomagnetic field paleomagnetic results from the most northerly areas of laurentia and europe i e ellesmere greenland and svalbard show clearly anomalous paleolatitudes because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain forest occurrences d v kent p e olsen earth planet sci lett 179 2000 311 324 we argue that long term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies estimates of the magnitude of the octupole dipole field ratio are not very precise but center around 0 1 which could cause errors in conventional paleopole determinations using the dipole formula of 7 5 because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid northern and mid southern hemisphere locations could thus be off by as much as 15 the well known misfit between the paleomagnetic results from the laurentia european and gondwana continents in a classical pangea a configuration could be explained by such errors due to octupole fields this explanation would negate the need to seek tectonic pangea b type solutions for the misfit these solutions have generally remained unacceptable in terms of the geology of the gondwana laurussia borderzone

diagenesis of sandstone reservoirs of pattani basin gulf of thailand miocene sandstones from gas fields in the gulf of thailand s pattani basin provide an example of rapid decline in porosity and permeability with increasing burial depth this decline results from rapid burial diagenesis that is related to very high geothermal gradients in the basin porosity loss with depth in these subquartzose sandstones results from mechanical compaction and from progressive cementation by quartz overgrowths kaolinite and illite quartz overgrowths increase with depth indicating continuous or episodic silica cementation kaolinite occurs as a pore filling cement and is abundant between the depths of 1 980 and 3 050 m 6 500 and 10 000 ft minor cements include calcite ankerite mixed layer illite smectite siderite pyrite chlorite and barite the best porosity and permeability in pattani basin reservoirs are generally associated with large intergranular pores in sandstones between 915 and 1 980 m 3 000 and 6 500 ft at greater depths most interparticle pores have been occluded and porosity is mainly secondary in origin skeletonized feldspars indicate progressive dissolution with increasing burial depth in low permeability sandstones from deeper zones 2 285 3 050 m or 7 500 10 000 ft porosity is mainly restricted to dissolution voids within detrital feldspars these secondary pores are usually partly filled by authigenic kaolinite and illite and their pore apertures are generally smaller 1 15 m than intergranular pore apertures 10 75 m however favorable reservoir properties may occur locally at de th where large feldspars have been leached from coarse grained sandstones

paleosecular variations recorded by holocene pleistocene sediments from lake el trebol patagonia argentina paleomagnetic studies carried out on six cores from the bottom sediments from lake el trebol south argentina are described measurements of directions declination and inclination d and i and intensity of natural remanent magnetisation nrm magnetic susceptibility at low and high frequency specific x and volumetric k isothermal remanent magnetisation irm saturation isothermal remanent magnetisation sirm back field and anhysteric remanent magnetisation arm were carried out stability of the nrm was investigated by alternating field demagnetisation rock magnetic studies suggest that the main carriers of magnetisation are ferrimagnetic minerals predominantly pseudo single domain magnetite records of nrm and k show a good correlation with the sedimentological description and also with those of shorter cores previously studied the tephra layers were identified from the lithologic profiles and also from the magnetic susceptibility logs due to their different chronological meaning and their rather bad behaviour as magnetic recorder these layers were removed from the sequence and the gaps that were produced along the profiles by the removal were closed obtaining a shortened depth radiocarbon age estimates from these cores and from earlier studies allow us to construct a paleosecular variation psv record for the past 24 000 years these results show good correlation with the previous one the spectrum obtained from spectral analysis shows major peaks at periodicities of about 1150 1500 2490 3630 5240 and 11 810 years both clockwise and counterclockwise precession of the geomagnetic vector is evident from analysis of the bauer plots with a preponderance of clockwise movement

chronology of the eocene continental deposits of africa magnetostratigraphy and biostratigraphy of the el kohol and glib zegdou formations algeria despite numerous discoveries that have considerably enriched the african arabian tertiary fossil record over the last decades our knowledge of the evolutionary history of many continental african vertebrate groups during the paleogene period remains inadequate particularly when it is compared with the fossil records of europe or north america the eocene epoch in africa is especially poorly documented being restricted to few fossiliferous localities our understanding of the early tertiary emergence diversification and paleobiogeographic history of african arabian mammals has been further hindered by the lack of a precise temporal framework for these sites we conducted magnetostratigraphic analyses associated with biostratigraphic studies in the fossiliferous sequences exposed in the northwestern hammadas of the saharan platform in the glib zegdou area and in the saharan atlas at the el kohol locality algeria to further define the age of these eocene continental deposits based on biostratigraphic constraints the six polarity zones identified in the el kohol section can be correlated with chrons c24n to c22r providing the first direct age estimates for the el kohol fossiliferous strata between 52 and 51 ma correlation to the geomagnetic polarity time scale using previously published biostratigraphic data for the glib zegdou fauna suggests an age ranging between 49 and 45 ma for this section the high resolution magnetostratigraphic study of the poorly known continental eocene epoch of algeria provides new insights into the early tertiary stratigraphy of northwest africa the placement of the algerian localities into a consistent chronological framework constitutes considerable advancement to achieve biostratigraphic correlation of the paleogene african arabian mammal localities

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

comparative radiocarbon dating of terrestrial plant macrofossils and aquatic moss from the ice free corridor of western canada in order to assess the reliability of aquatic moss for radiocarbon dating sup 14 c analyses were performed on a stratigraphic series of terrestrial plant macrofossils and samples of drepanocladus crassicostatus from a small hard water lake ph 8 2 in the ice free corridor of alberta all sup 14 c dating was done by using accelerator mass spectrometry mazama ash provided an independent chronological control the aquatic bryophyte samples consistently produced sup 14 c ages significantly older than the terrestrial macrofossils the relation between the radiocarbon dates from the macrofossils and the moss was not linear and age differences ranged from approximately 1400 to 6400 yr the sup 14 c content of d crassicostatus growing in the lake at present was less than 85 modern despite the apparent inability to take up sup 14 c deficient carbon by the direct incorporation of bicarbonate the bryophytes clearly do not provide reliable material sup 14 c dating the sup 14 c deficiency of aquatic mosses may be explained by the generation of sup 14 c deficient co sub 2 through isotopic exchange the formation of co sub 2 from bicarbonate by chemical processes and metabolic co sub 2 production these results demonstrate the potential unreliability of sup 14 c dates from aquatic mosses and raise serious concernsmore about the deglaciation dates from the ice free corridor that were obtained from aquatic drepanocladus less

advances in grain size mineral and pore scale characterization of lithic and clay rich reservoirs clay and lithic rich sandstones are difficult to characterize through uncored well sections in terms of their grain size porosity and mineralogy all of which are required for assessing reservoir quality and production performance this paper presents results from a study through one such interval and shows how a combination of different techniques can be used to better understand rock properties of complex reservoirs thereby helping to reduce reservoir uncertainty in this study mean data from laser grain size analysis are comparable to point counted grain size and both are considered as viable analytical methods automated quantitative evaluation of minerals by scanning electron microscopy qemscan provides a further useful and consistent grain size measurement that can be applied to both core and cuttings samples the qemscan has also proved to be a valuable technique in the mineralogical analysis of sandstones that are lithic clay and feldspar rich eliminating the subjective nature that is inherent with optical analysis results from the studied interval show that porosity measured by conventional core analysis ca and mercury injection capillary pressure micp analysis are generally comparable with log derived total porosity porosity measured from point counting and qemscan techniques is significantly lower than total porosity with the qemscan porosity locally equivalent to log derived effective porosity both point count and qemscan porosities show better correlations with permeability and 0 94 respectively than total porosity values and 0 60 ca and micp respectively suggesting that they might provide a measure of effective porosity in high quality reservoir rocks

paleomagnetic evidence for a stationary marion hotspot additional paleomagnetic data from madagascar abstract the island of madagascar experienced widespread magmatism at ca 90 ma due to its interaction with the marion hotspot previous paleomagnetic data from igneous rocks in the southwestern and northwestern regions of the island indicated that the marion hotspot has remained fixed for the past 90 ma we report paleomagnetic data from northeastern madagascar d analava complex samples were collected from basalts rhyolites gabbros and a dolerite dyke sixty samples from 5 sites yield a paleomagnetic pole at 66 7 s 43 5 e a95 10 7 and a grand mean pole gmp calculated from 10 different studies covering the entire island of madagascar falls at 68 9 s 49 0 e a95 4 4 this pole translates to a paleolatitude for the volcan de l androy focal point of the hotspot at 45 2 6 5 s compared to the current location of the marion hotspot at 46 s our results confirm and expand upon previous studies that argue for the fixity of the marion hotspot for the past 90 ma

determination of petrographic factors controlling permeability using petrographic image analysis and core data satun field pattani basin gulf of thailand abstract miocene sandstones of the pattani basin gulf of thailand undergo progressive diagenesis with depth permeability varies from the millidarcy to darcy range over the short depth interval of 6000 9000 ft 1828 2748 m with a tendency for deeper samples to be less permeable image analysis of representative thin sections showed that these rocks contain five pore types three of which are strongly related to permeability at shallower depths less than 7500 ft 2285 m the relative abundance of the two largest pore types are major controls on enhanced permeability below 7500 ft 2285 m the median throat sizes of these pore types are halved 10 versus 20 m and permeability is greatly reduced a small intergranular pore type unimportant in the upper section has an increased throat size from about 2 to 6 m below 7500 ft 2285 m physical models of permeability allow the construction of artificial rocks where the contributions to permeability of each pore type are calculated at various porosity values these models can be used to assess the maximum permeability likely to be encountered even if such maxima are not observed in core

implications of a nonlinear 40ar 39ar age progression along the louisville seamount trail for models of fixed and moving hot spots 1 the louisville seamount trail has been recognized as one of the key examples of hot spot volcanism comparable to the classic volcanic hawaiian emperor lineaments the published total fusion 40ar 39ar data of watts et al 1988 showed an astonishing linear age progression firmly establishing louisville as a fixed hot spot in the south pacific mantle we report new 40ar 39ar ages based on high resolution incremental heating 40ar 39ar dating for the same group of samples showing a marked increase in both precision and accuracy one of the key findings in our reexamination is that the age progression is not linear after all the new data show a significantly decreased apparent plate velocity for the louisville seamount trail older than 62 ma but confirm the linear trend between 47 ma and the present day although based on only three samples over 2150 km the most recent volcanic activity in the louisville seamount trail has now been dated at 1 11 0 04 ma for the most southeastern seamount located at 50 26 s and 139 09 w these results indicate that the louisville age progression should be interpreted on the basis of both plate and hot spot motion in this paper we examine our new results in conjunction with the numerical mantle flow models of steinberger et al 2004 that also predict marked deviations from simple linear age progressions with these models we can achieve a good fit to the geometry of both the hawaiian and louisville seamount trails and their age progressions as well as the 15 paleolatitudinal shift observed by tarduno et al 2003 for the hawaiian hot spot between 80 and 47 ma if the model is restricted to pacific hot spots only we can improve the fit to the nonlinear age trend for the louisville seamount trail by allowing an additional rotation change of the pacific plate around 62 ma and by decreasing the initiation age of the louisville plume from 120 to 90 ma this improved model features a significant eastward hot spot motion of 5 between 80 and 30 ma for the louisville hot spot which is quite dissimilar to the southward motion of the hawaiian hot spot during the same time interval followed by a minor 2 latitudinal shift over the last 30 myr if hot spot tracks are considered globally the age trend observed for the oldest part of the louisville seamount trail does not entirely follow the numerical model predictions this may indicate some remaining inaccuracies in the global plate circuit but it may also indicate that the louisville hot spot experienced a motion somewhat different than in the numerical model faster in the interval between 62 and 47 ma but slower before that

cretaceous reconstructions of east antarctica africa and madagascar abstract some cretaceous reconstructions of east antarctica africa and madagascar overlap the mozambique ridge onto the margin of dronning maud land and others place madagascar west of the gunnerus ridge into the riiser larsen sea our m10ny 131 9 ma m0 120 4 ma 96 ma and 34y 83 ma marine free air gravity and topography reconstructions illustrate a solution to the overlap problem and also demonstrate that madagascar separated from antarctica east of the gunnerus ridge our reconstructions chronicle the growth of the conjugate mozambique and riiser larsen basins created at the southwest indian spreading ridge well defined spreading corridors identified by conjugate fracture zones and mesozoic magnetic anomaly identifications extend from one basin into the other we find that the eastern scarp of the mozambique ridge aligns with the astrid ridge and there is no room for madagascar west of the gunnerus ridge the overlap of the mozambique ridge is solved by taking into account the motion of an independent mozambique ridge microplate that existed from chron m11 to circa m2

the last interglacial climate during the last 10 000 years the holocene has been relatively mild and stable in contrast the climate during the last interglacial is often portrayed as more variable but as kukla discusses in this perspective evidence for a more stable last interglacial is emerging furthermore the transition to the next glacial proceeded in stages and was not uniform across europe

jurassic paleopole controversy contributions from the atlantic bordering continents the early early middle jurassic and early cretaceous mean paleopoles for north america europe south america and africa are very well grouped in appropriate continental reconstructions but the intervening late middle and late jurassic segment of the apparent polar wander path apwp is poorly defined and controversial the available paleopoles reconstructed for the partial opening of the central atlantic ocean form a scattered grouping with no coherent age patterns illustrating that they do not constitute a robust data set uncertainties in the reconstruction parameters between north america and europe also play a role however excellent paleomagnetic results exist for tectonic elements near the margin of west gondwana that are unlikely to have been significantly displaced with respect to cratonic africa and south america these results have not previously been used for apwp reconstructions because local rotations are thought to have deflected the paleopoles in many cases the inclinations of such results however can be used to determine a locus of paleopole positions paleopole loci for about 150 and 170 ma were determined from results from spain italy lebanon and the chilean andes and these were rotated with appropriate parameters to give locus intersections in north american coordinates a late middle jurassic early callovian best estimate of the paleopole in north american coordinates is located at about lat 70 n long 135 e and a late jurassic kimmeridgian best estimate is located at about 70 n 155 e the resulting jurassic early cretaceous apwp follows roughly the 70th parallel passing through the middle of the scattered individual paleopoles from the cratonic parts of the atlantic bordering continents

geoframework coupling multiple models of mantle convection within a computational framework solver coupling can extend the capability of existing modeling software and provide a new venue to address previously intractable problems a software package has been developed to couple geophysical solvers demonstrating a method to accurately and efficiently solve multiscale geophysical problems with reengineered software using a computational framework pyre pyre is a modeling framework capable of handling all aspects of the specification and launching of numerical investigations we restructured and ported citcoms a finite element code for mantle convection into the pyre framework two citcoms solvers are coupled to investigate the interaction of a plume at high resolution with global mantle flow at low resolution a comparison of the coupled models with parameterized models demonstrates the accuracy and efficiency of the coupled models and illustrates the limitations and utility of parameterized models

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

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

late cretaceous cenozoic deformation of northeast asia abstract the plate tectonic paradigm implies rigid plates and narrow plate boundaries in contrast diffuse plate boundaries are common both in the oceans and continents r g gordon annu rev earth planet sci 26 1998 615 642 and their history is difficult to constrain especially in remote tectonically complex areas such as northeast asia m e chapman s c solomon j geophys res 81 1976 921 930 here we show how extensive north atlantic marine magnetic r macnab et al eos 76 1995 449 458 and gravity data d t sandwell w h f smith j geophys res 102 1997 10039 10054 can be used to unravel with tight confidence limits successive periods of deformation over 80 million years along the diffuse continental eurasian north american plate boundary a period of compression in the late cretaceous 14 mm yr in the laptev sea to 20 mm yr in kamchatka led to thrusting in the verkhoyansk mountains and was followed by extension from 68 to 40 ma when 400 km of extension was accommodated by the formation of a series of grabens including the moma rift system since 40 ma time varying compression and transpression along the moma rift system created strike slip faults thrusts and folds at rates up to 6 3 mm yr in the laptev sea region 600 km of extension from latest late cretaceous to present created the laptev sea and lena rift systems the deformation predicted by our model fits most geological features formed in the laptev sea and central northeast asia during late cretaceous cenozoic times the most recent deformation late miocene pliocene is not very well constrained since our model lacks data younger than 11 ma the deformation that occurred in kamchatka reflects a complex tectonic setting and our model s predictions are only tentative

postglacial vegetation climate and fire history along the east side of the andes lat 41 42 5 s argentina abstract the history of the low elevation forest and forest steppe ecotone on the east side of the andes is revealed in pollen and charcoal records obtained from mid latitude lakes prior to 15 000 cal yr bp the vegetation was characterized by steppe vegetation with isolated stands of nothofagus the climate was generally dry and the sparse vegetation apparently lacked sufficient fuels to burn extensively after 15 000 cal yr bp a mixture of nothofagus forest and shrubland steppe developed fire activity increased between 13 250 and 11 400 cal yr bp contemporaneous with a regionally defined cold dry period huelmo mascardi cold reversal the early holocene period was characterized by an open nothofagus forest shrubland mosaic and fire frequency was high in dry sites and low in wet sites the data suggest a sharp decrease in moisture eastward from the andes a shift to a surface fire regime occurred at 7500 cal yr bp at the wet site and at 4400 cal yr bp at the dry site preceding the expansion of austrocedrus by 1000 1500 yr the spread of austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late holocene the change to a surface fire regime is consistent with increased interannual climate variability and the onset or strengthening of enso the present day mixed forest dominated by nothofagus and austrocedrus was established in the last few millennia

the origin of intraplate deformation in the atlas system of western and central algeria from jurassic rifting to cenozoic quaternary inversion abstract analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central saharan atlas algeria this intraplate area has recorded the breakup of pangea late triassic the opening of the maghrebian tethys since the dogger and subsequently its closure oligocene to present however the two periods of strong coupling between europe and africa late lutetian and pleistocene which correspond to rapid uplifts of the atlas system and important deformations are not collision related they can be correlated to the beginning and the end of the development of the western mediterranean sea

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

ash resuspension related to the 2011 2012 cordon caulle eruption chile in a rural community of patagonia argentina abstract the 2011 2012 cordon caulle eruption emitted about 1 km3 of rhyodacitic tephra dominant westerly winds in the region caused most of the primary tephra to deposit in neighboring argentina in addition to the impact of widespread dispersal and fallout of primary tephra during the eruption argentina was also significantly affected by remobilization of the primary ash even several years after the climactic phase of the eruption in this mixed methods study we combine aspects of natural and social sciences to characterize the ash resuspension events associated with the 2011 2012 cordon caulle deposits and assess the impacts on the argentinian farming community of ingeniero jacobacci in the patagonian steppe our findings show the primary importance of wind rainfall and ash availability in controlling the occurrence and persistence of ash resuspension events the role played by these variables was also reflected in the seasonal distribution of events observed regarding the impacts our results complement those of earlier studies and demonstrate that ash resuspension events can exacerbate the negative impact of primary tephra fallout events from the time of deposition to many years after the eruption only after five years has the environment and the farming community begun to show signs of recovery our findings also highlight the importance of assessing ash resuspension events in multi hazard scenarios involving volcanic and hydrometeorologic hazards

magnetostratigraphy and biostratigraphy of the neogene deposits of kastellios hill central crete greece abstract the kastellios hill section is situated in central crete and composed mainly of mudstones and silts deposited in lacustrine brackish or estuarine environments several levels of this section have yielded the remains of mammals foraminifera ostracoda pollen and spores the palynological analysis shows that the region was forested in the upper miocene with a vegetation confined to climates warmer than the present the age indicated by foraminifera is early tortonian while the mammals were attributed to late vallesian in continental biochronology of this section 88 m have been sampled for a magnetostratigraphic study several experiments have been made on the magnetic properties of these sediments showing that they are suitable for a paleomagnetic study moreover x ray diffraction and microprobe analysis of 19 samples regularly distributed over the section show that no major changes occurred in the chemical composition from 77 separate sites 241 samples have been demagnetized thermally 215 or by alternating field 26 the main result of this study is the definition of a long reversed polarity period interrupted by two short normal intervals this magnetostratigraphic sequence can be correlated using biostratigraphic data with a part of chron 5 r of the geomagnetic time scale in other words this section is probably younger than 11 47 my and older than 10 30 my

magnetic polarity stratigraphy and ages of siwalik group rocks of the potwar plateau pakistan abstract six magnetic polarity sections have been established over the potwar plateau region of pakistan including the major stratotypes of the siwalik group in all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone which is contained within the nagri formation this conspicuous normal polarity zone has been radiometrically dated at 9 5 0 6 m y which identifies it as magnetic chron 9 radiometric dates from the upper siwalik formation have also been used to identify the chron 2 3 boundary in two of the sections the magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale so that the ages of the local stratigraphy are indexed accordingly based exclusively on data from stratotype sections the chinji nagri and dhok pathan formations have nominal age ranges of 10 1 13 1 7 9 10 1 and 5 1 7 9 m y age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype mean sediment accumulation rates during the chinji nagri and dhok pathan interval range from 13 to 52 cm 103 yr essentially linear sediment accumulations are locally maintained over time intervals of several million years the chinji nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the potwar plateau indicating a fundamental sedimentary tectonic change at this time

first carbon isotope chemostratigraphy of the ouled abdoun phosphate basin morocco implications for dating and evolution of earliest african placental mammals abstract the well known maastrichtian ypresian vertebrate bearing phosphate series in the ouled abdoun basin morocco is classically dated using regional selachian biostratigraphic zonation these marine sediments yielded paleocene and eocene mammals comprising the earliest known placentals from africa this study provides the first insight into the organic carbon isotope chemostratigraphy 13 c org of the moroccan phosphate series and a refined dating of its vertebrate bearing levels four paleocene eocene sections in the ne ouled abdoun quarries show consistent 13 c org long term evolutions from the base to the top 1 positive trend in phosphorite bed iia beginning with the lower bone bed yielding mammals such as eritherium ocepeia abdounodus lahimia of early thanetian and selandian age 2 transitional negative trend in the intercalary phosphorite beds ii i that includes the otodus obliquus and phosphatherium escuilliei bone bed of earliest ypresian age 3 negative trend to the lowermost 13 c org values that are correlative to the early middle ypresian interval including etm 2 and etm 3 hyperthermal events in the global record 4 positive trend in chert enriched facies containing the middle ypresian eeco global climatic event our chemostratigraphic study of the ouled abdoun phosphate series provides a new chronostratigraphic framework for calibrating the beginning of the evolution of placental mammals in africa the lower bone bed level from the paleocene phosphorite bed iia yielding eritherium is not younger than early thanetian and is most likely selandian the phosphatherium bone bed in the intercalary beds ii i is earliest ypresian the phosphorite bed 0 from which daouitherium probably came is early middle ypresian just below the eeco this suggests that the first large proboscideans evolved after the petm during mid ypresian warming events the 13 c org study does not support the presence of lutetian in the ne ouled abdoun phosphate series and suggests that a noticeable part of the upper thanetian is absent

synchronisation of sedimentary records using tephra a postglacial tephrochronological model for the chilean lake district abstract well characterised tephra horizons deposited in various sedimentary environments provide a means of synchronising sedimentary archives the use of tephra as a chronological tool is however still widely underutilised in southern chile and argentina in this study we develop a postglacial tephrochronological model for the chilean lake district ca 38 to 42 s by integrating terrestrial and lacustrine records tephra deposits preserved in lake sediments record discrete events even if they do not correspond to primary fallout by combining terrestrial with lacustrine records we obtain the most complete tephrostratigraphic record for the area to date we present glass geochemical and chronological data for key marker horizons that may be used to synchronise sedimentary archives used for palaeoenvironmental palaeoclimatological and palaeoseismological purposes most volcanoes in the studied segment of the southern volcanic zone between llaima and calbuco have produced at least one regional marker deposit resulting from a large explosive eruption magnitude 4 some of which now have a significantly improved age estimate e g the 10 5 ka llaima pumice eruption from llaima volcano others including several units from puyehue cordon caulle are newly described here we also find tephra related to the cha1 eruption from chaiten volcano in lake sediments up to 400 km north from source several clear marker horizons are now identified that should help refine age model reconstructions for various sedimentary archives our chronological model suggests three distinct phases of eruptive activity impacting the area with an early to mid holocene period of relative quiescence extending our tephrochronological framework further south into patagonia will allow a more detailed evaluation of the controls on the occurrence and magnitude of explosive eruptions throughout the postglacial

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

paleogeographic and paleoenvironmental variations in the area of the pueyrredon posadas and salitroso lakes santa cruz province argentina during the holocene and its relationship with occupational dynamics abstract this work constitutes an integration of geological archaeological and palynological data from santa cruz province argentina for assessing the influence of paleogeographic and paleoclimatic changes in relation to the dynamics of human occupation during the holocene in order to carry out its objectives we surveyed different archaeological sites we collected pollen samples from two archaeological sites and described sampled and dated stratigraphic profiles of lake levels a digital elevation model was also developed to reconstruct old flooded areas given the obtained results we observe the influence that paleogeographic changes could have had on human occupational dynamics these changes may have opened up new areas for both settlement and mobility pollen records suggest a vegetation change from grass steppe to shrub steppe during the middle holocene most likely related to climate changes identified at a regional scale

some geometrical aspects of uncertainties in combined plate reconstructions the uncertainties in both the position of a pole of rotation and the rotation angle describing a plate reconstruction depend not only on the number and quality of data used but also on two geometrical factors the length of plate boundary represented by the data and the distance from the best fit pole to the center of the data region we describe a simple geometrical method that can be used to calculate minimum uncertainties in reconstructions based on positions of magnetic anomalies and fracture zones with finite uncertainties uncertainties in pole positions and angles corresponding to 10 km uncertainty in individual data points can when combined yield uncertainties in reconstructed positions greatly in excess of 10 km per rotation for example an uncertainty of 10 km in anomaly 6 reconstructions in the south pacific southeast indian northwest indian and north atlantic oceans would result in uncertainty of up to 190 km in the reconstructed position of the pacific plate with respect to north america at 19 8 m y b p existing data show that the realistic uncertainties for this case are twice as large as the calculated minimum uncertainties

fracture zones in the north atlantic from combined geosat and seasat data we present a new tectonic fabric map of the northern and central north atlantic ocean floor from 10 s to 66 n based on combined geosat and seasat altimetry data short wavelength vertical gravity along track was derived by hilbert transforming and high pass filtering geosat and seasat deflection of the vertical profiles the along track gravity data were used in conjunction with magnetic anomaly data to locate a large number of fracture zones as well as extinct spreading ridges and v shaped structures we traced many fracture zones more continuously and closer to adjacent continental margins than was previously possible we also mapped a number of unnamed fracture zones and fracture zone segments many of which were previously uncharted we have evaluated the agreement between the mapped fracture zones and synthetic flow lines as predicted from recent plate models for the opening of the north central north and south atlantic most fracture zones in this area correlate quite well with synthetic flow lines supporting the proposition that geosat seasat altimetry data serve as a useful tool to map fracture zones where insufficient bathymetric data are available many fracture zones including some small offset fracture zones in the central north atlantic have been stable and follow tectonic flow lines for tens of millions of years however many other fracture zones do not reflect a railroad track pattern of plate motions the geometry of many small and medium offset fracture zones indicates that the ridge axis discontinuities that produce them can migrate along the ridge causing off axis traces that deviate from tectonic flow lines velocity space diagrams constructed using a recent relative and absolute present day plate motion model indicate that some low angle v shaped troughs and ridges are likely formed by migration of the plate boundary relative to a mesospheric reference frame other low angle v shaped structures reflect slow migrations oscillations of small offset ridge axis discontinuities that are distinct from both tectonic flow lines and v shapes reflecting absolute plate motions high angle v shaped structures are the result of fast migrating ridge axis discontinuities propagating rifts

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

paleolacustrine records from late pleistocene holocene in the perito moreno national park argentinian patagonian andes abstract the reconstruction of glacial paleolake formation and evolution allows the understanding of glacier advance and retreat behavior for longer periods of time new datings obtained from paleolacustrine deposits in the argentinian patagonian andes northwestern region of santa cruz province reveal that glacial lakes formed before the last glacial maximum they being as old as 31 278 cal yr bp and evolved until 6900 cal yr bp the interpretation of stratigraphic profiles described in different sectors of the perito moreno national park pmnp allows reconstructing lacustrine glaciolacustrine glacial till and delta paleoenvironments in each sector organic rich sediments were also sampled to obtain four absolute ages through ams methodology thus the paleogeographic reconstructions proposed are based on paleobathymetries performed from digital elevation models datings and facies distribution of lacustrine deposits mainly composed of laminated silts with subordinated sands and muds based on these new data the paleogeography of this area is characterized by two lakes at 880 m a s l at 31 200 ca yr bp one directly related to modern belgrano lake while the other is probably related to the burmeister lake but more than 10 km toward the east as respect to its current position the paleogeography reconstruction shows between 11 731 cal yr bp and 10 819 cal yr bp the existence of a major paleolake at 900 920 m a s l which was connected to several modern lakes finally a decrease of the paleolake level between 900 and 890 m a s l was recorded at 6900 cal yr bp which produced two major lacustrine systems separated by moraine deposits

changes of the earth s rotation axis owing to advection of mantle density heterogeneities polar wander the secular motion of the earth s rotation axis relative to its surface has been studied for many years dynamical arguments1 3 show that polar wander can arise from the redistribution of mass in a plastic deformable earth the rate depending on both the rate of mass redistribution and the rate at which the earth s rotational bulge can readjust to the changing rotation axis here we use a viscosity structure obtained through geoid modelling4 a mantle flow field consistent with tomographic anomalies5 and time dependent lithospheric plate motions6 to calculate the advection of mantle density heterogeneities and corresponding changes in the degree two geoid during the cenozoic era we show that the rotation axis will follow closely any imposed changes of the axis of maximum non hydrostatic moment of inertia the resulting path of the rotation axis agrees well with palaeomagnetic results7 with the model predicting a current rate of polar motion that explains 40 of that observed geodetically8

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

ground based measurements of spatial and temporal variability of snow accumulation in east antarctica the east antarctic ice sheet is the largest highest coldest driest and windiest ice sheet on earth understanding of the surface mass balance smb of antarctica is necessary to determine the present state of the ice sheet to make predictions of its potential contribution to sea level rise and to determine its past history for paleoclimatic reconstructions however smb values are poorly known because of logistic constraints in extreme polar environments and they represent one of the biggest challenges of antarctic science snow accumulation is the most important parameter for the smb of ice sheets smb varies on a number of scales from small scale features sastrugi to ice sheet scale smb patterns determined mainly by temperature elevation distance from the coast and wind driven processes in situ measurements of smb are performed at single points by stakes ultrasonic sounders snow pits and firn and ice cores and laterally by continuous measurements using ground penetrating radar smb for large regions can only be achieved practically by using remote sensing and or numerical climate modeling however these techniques rely on ground truthing to improve the resolution and accuracy the separation of spatial and temporal variations of smb in transient regimes is necessary for accurate interpretation of ice core records in this review we provide an overview of the various measurement techniques related difficulties and limitations of data interpretation describe spatial characteristics of east antarctic smb and issues related to the spatial and temporal representativity of measurements and provide recommendations on how to perform in situ measurements

petrography and reservoir physics i objective classification of reservoir porosity porosity observed in thin section can be objectively classified using a combination of digital acquisition procedures and pattern recognition algorithms pore types are derived from the frequency distributions of sizes and shapes of patches of porosity exposed in thin section each pore type is represented by a characteristic distribution of sizes and shapes found in thin section most sandstone reservoirs contain fewer than six pore types much of the intersample variability in reservoir physics is associated with changes in pore type abundance the advantages of this approach to porosity classification are 1 the criteria for classification are objectively defined 2 classification procedure is rapid accurate and precise 3 pore types are understood easily in ter s of conventional genetic classification schemes and 4 pore type data are related strongly to petrophysical properties

the uncertainties of finite rotations in plate tectonics incomplete knowledge of the pattern of magnetic lineations and fossil transform faults represented by sea floor spreading data on two plates generated by the same spreading center leads to uncertainties in a reconstruction of the past relative configuration of the plates a reconstruction may be represented by the finite rotation that describes one configuration relative to another in this paper a method of reconstruction is provided that reflects the uncertainties in the data the method minimizes a weighted least squares measure of fit as a function of the rotation parameters for a given rotation the measure of fit represents the sum of squares of the weighted distances of fixed and rotated data points representing corresponding former plate margin segments from a common plate margin the common margin is estimated from the locations of both fixed and rotated data points the estimated margin consists of a set of great circle arcs it is shown how the method of reconstruction may be utilized to obtain an uncertainty region for the pole and angle of rotation that characterize the finite rotation these techniques are then used to study the uncertainties of reconstructions in the south pacific for the times of anomalies 13 and 18 the best fit reconstructions and the uncertainty region for the anomaly 18 pole are in good agreement with previous work however the uncertainties in the anomaly 13 pole were substantially underestimated by previous investigators

paleomagnetic evidence for motion of the hawaiian hotspot during formation of the emperor seamounts the bend in the hawaiian emperor chain is the best example of a change in plate motion recorded in a fixed hotspot frame of reference alternatively the bend might record primarily differences in motion of the hawaiian hotspot relative to the pacific lithosphere new paleomagnetic data from the emperor chain support the latter view althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain the best available paleomagnetic data suggest pacific hotspots may have moved at rates comparable to those of lithospheric plates 30 mm yr 1 in late cretaceous to early tertiary times 81 43 ma if correct this requires a major change in how we view mantle dynamics and the history of plate motions in the early to mid cretaceous 128 95 ma hotspots in the atlantic moved at similar rates these episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion such as the past 5 m y

paleointensity studies on holocene pleistocene sediments from lake escondido argentina abstract relative changes in geomagnetic field intensity over the last 16 000 years bp were recovered from the study of four cores obtained from lake escondido south argentina rock magnetic analysis shows that the main magnetic mineral is magnetite the concentrations being mainly between 0 01 and 0 1 and the grain size between 1 and 8 m in order to fulfil the criteria for assessing the reliability of paleointensity data derived from sediments rev geophys 31 1993 319 the samples whose concentrations and size are beyond this range were rejected the remanent magnetisation at 20 mt nrm20 mt was normalised using the anhysteric remanent magnetisation arm100 mt the saturation of the isothermal remanent magnetisation sirm and the low field magnetic susceptibility k coherence function analysis indicates that the records are not significantly affected by local environmental conditions this suggests that the variations in normalised remanence are mostly likely due to geomagnetic paleointensity fluctuations arm100 mt is the best normaliser for the lake escondido sediments the record of relative paleointensity nrm20 mt arm100 mt shows peaks and troughs whose amplitudes are similar to those in the st lawrence estuary lake baikal lake pepin and larsen a ice shelf data set and a stack of archeomagnetic data given as virtual axial dipole moment vadm

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

late pleistocene palaeolakes in the andean and extra andean patagonia at mid latitudes of south america abstract late pleistocene palaeolakes on the eastern side of andes glacial lakes and in the patagonian steppe pluvial lakes along 41 s latitude were studied natural dams glaciers moraines that maintained a high water level in glacial palaeolakes were melted or cut by erosion during late pleistocene climate amelioration with the retreat of the glaciers from the andean valleys during prominent regional deglaciation outflow from the eastern side of andes to the pacific ocean was formed about 13 2 ka bp as a result the water level in glacial palaeolakes fell rapidly in stages these changes were often preceded by volcanic tectonic activity which is suggested to be the touch of butterfly releasing mass balance adjustment in the changing landscape the lacustrine tephra stratigraphic pattern from the andes comprising more than 50 horizons since the last glaciation is presented the correlation of palaeoenvironmental changes between the andes and the patagonian steppe is suggested on the basis of two dacite marker horizons the deglaciation event that led to development of the new hydrological system in the andean palaeolakeland formed several large but short lived palaeolakes in closed basins in the patagonian steppe probably due to melting of local ice and permafrost accumulated during glaciation on the surrounding mountains and elevated plains