slow but not fast global expansion may explain the surface dichotomy of earth abstract various fast and slow earth expansion hypotheses are reviewed principal objectives are 1 1 to aid geologists to overcome their current mysticism in rejecting accepting or sympathising with earth expansion hypotheses without distinction and 2 2 to stimulate geophysicists into taking full account of the possibility that slow earth expansion by core mantle interaction may have occurred inward migration of the core mantle boundary due to the differentiation and progressive solidification of the liquid outer core might have caused expansion of the earth s radius at a slow mean rate of 0 5 mm a 1 a slowly expanding earth is attractive since it could explain why two distinct types of lithosphere exist oceanic and continental

comparison between the hot spot and geomagnetic field reference frames hot spots are located on the earth s surface and are caused by hot plumes of upwelling mantle material they manifest themselves by volcanic activity which changes its location in response to the movement of the lithosphere over the mantle plume it has been suggested1 3 that hot spots are relatively stationary with respect to each other and also that they represent a frame of reference with which to measure the absolute movement of the lithospheric plates over the surface of the earth another frame of reference can be partially defined by the position of the mean magnetic dipole field of the earth as the dipolar field is believed to be closely related to the spin axis this frame of reference automatically gives information about the palaeolatitude and orientation of the lithospheric plates but since the position of the ancient dipole field of the earth does not give any information about palaeolongitude and since in many cases the definition of the field is relatively poor due to less than adequate rock collections during the time of interest hot spot reference frames have been favoured by many people we now report a comparison of the two frames of reference and show that over the past 200 myr there has been relative movement between the two frames of reference of about 20 and that this motion has not been constant with time

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research

comments on a weighted least squares fit of the australian apparent polar wander path for the last 100 myr by r j musgrave with the recent publication of musgrave s paper musgrave 1989 two late mesozoic cainozoic pole paths that differ substantially in age calibration have become available for australia the earlier path path 1 is age calibrated almost entirely by remanence directions of sedimentary sequences idnurm 1985 the new path path 2 is calibrated by the remanence directions of volcanics path 2 has the special merit that again it puts to use data which fell out of favour in the late 1970s and early 1980s because of serious discrepancies with the indian data base e g embleton 1981 embleton mcelhinny 1982 the two approaches to australia s late mesozoiccainozoic apwp differ fundamentally in the treatment of scatter in pole data the approach adopted for path 1 attempts to eliminate abnormally large scatters at the source believed to be the incomplete averaging out of geomagnetic secular variations for discussion see idnurm 1985tby utilizing the time averaging characteristics of sedimentary sequences the approach for path 2 relies heavily on statistics to eliminate the scatter the differences between the paths are of concern especially when they are used for age dating of weathered profiles the age estimates may differ by up to about 15ma in the tertiary corresponding to an angular difference of so and musgrave has attempted to test the paths by comparing their respective age estimates on such units with independent geological constraints the results are in favour of path 2 but he appears to have made some errors in these comparisons errors that invalidate the finding more seriously the method devised for age calibration of path 2 seems unsound the first error occurs when the palaeomagnetic age estimates for a new caledonian laterite are compared with stratigraphic evidence that its age is early miocene or older musgrave finds that this stratigraphic age limit is consistent with path 2 but not consistent with path 1 and concludes that the test has provided a clear discrimination in favour of the new apwp this is not correct fig 1 shows path 1 with the new caledonian pole aa since the 95 per cent confidence circle for aa encloses the 26 ma calibration pole path 1 is consistent with an early miocene age for the laterite as a separate issue all three new caledonian laterite poles in musgrave s paper have values of a that are excessively high for such material see for example his table 1 suggesting unresolved complexities in their remanence

the time averaged paleomagnetic field we review indications of persistent deviations from the geocentric axial dipole model of the time averaged geomagnetic field and present a zonal harmonic model derived from 185 deep sea sediment piston cores taken from low to middle latitudes to approximately 45 analysis of the paleomagnetic inclination recorded in these cores for the brunhes normal polarity 0 73 ma and matuyama reverse polarity 0 73 2 47 ma chrons after plate motion correction gives well constrained estimates of the dominant long term nondipole contributions the axial quadrupole and axial octupole and shows no significant deviation from axial symmetry the amplitude of the axial quadrupole is found to vary with polarity 2 6 of the geocentric axial dipole for normal 4 6 for reverse while the axial octupole does not show appreciable change 2 9 for normal 2 1 for reverse these estimates of the quadrupole contribution agree well with prior determinations for the plio pleistocene 0 5 ma however the octupole contribution we find is opposite in sign to previous estimates we suggest that a negative octupole is representative of the actual time averaged paleomagnetic field while prior positive octupole estimates probably reflect spurious inclination shallowing the lack of polarity asymmetry in the octupole suggests that this nondipole component may be more closely linked to the main dipole field than is the quadrupole and so supports models of the geodynamo in which dipole and quadrupole families do not interact

volatile emission during the eruption of baitoushan volcano china north korea ca 969 ad magma volume dre 24 5 km3 the main phase ca 95 vol is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites the eruption column is estimated to have reached ca 25 km and thus entered the stratosphere a late phase 5 vol is represented by trachyte emplaced chiefly as moderately welded ignimbrites the comendites contain 3 and the trachytes 10 20 vol phenocrysts mainly anorthoclase hedenbergite and fayalite primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra whereas those in phenocrysts in the trachytes are devitrified the comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra the mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method the average h2o concentration of the comenditic matrix glass is 1 5 wt probably largely secondary and of the corresponding melt inclusions 5 2 wt melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm the comenditic matrix glasses are represented by a fluorine rich 3992 ppm f and fluorine poor group 2431 ppm f averaging 3853 ppm for chlorine only 20 of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of 250 ppm s the difference between pre and post eruptive concentration of h2o is at least 3 7 0 6 wt h2o taking into consideration re hydration of the matrix glass and possible leakage of melt inclusions the difference between pre and post eruptive concentrations of the halogens amounts to 909 90 ppm cl and 1863 280 ppm and 302 40 ppm f the difference for s was estimated based on the average of the maximum s concentrations in the melt inclusions 455 ppm s and the detection limit resulting in 205 40 ppm s the calculated mass of volatiles injected into the atmosphere based on the erupted magma volume and volatile data is 1796 453 megatons for h2o 45 10 megatons for chlorine 42 11 megatons for fluorine and 2 0 6 megatons for sulfur the 969 20 ad eruption of baitoushan volcano one of the largest eruptions of the past 2000 years is thought to have had a substantial but possibly short lived effect on climate

refining the eruptive history of ulleungdo and changbaishan volcanoes east asia over the last 86 kyrs using distal sedimentary records abstract the eruptive histories of ulleungdo south korea and changbaishan north korea china border volcanoes are not well constrained since their proximal stratigraphies are poorly exposed or largely inaccessible however determining the past behaviour of these volcanoes is critical since future eruptions are likely to disperse ash over some of the world s largest metropolitan regions alkaline tephra deposits erupted from both centres are routinely identified in marine cores extracted from the sea of japan as well as high resolution lacustrine records east of the volcanoes here we review the distal ash occurrences derived from ulleungdo and changbaishan and provide new data from the lake suigetsu central honshu japan sediment core in order to provide a more complete and constrained eruption framework the intensely dated lake suigetsu archive provides one of the most comprehensive distal eruption records for both centres despite being located ca 500 km e of ulleungdo and ca 1000 km sse of changbaishan the suigetsu record is utilised to precisely date and geochemically fingerprint using major minor and trace element glass compositions ash fall events that reached central honshu here we identify a new non visible cryptotephra layer in the suigetsu sediments which reveals a previously unreported explosive event from changbaishan at 42 750 42 323 intcal13 yrs bp 95 4 confidence interval this event is chronologically and geochemically distinct from the b j baegdusan japan basin tephra reported in the sea of japan ca 50 ka furthermore we also confirm that the widespread u ym tephra erupted from ulleungdo reached central japan and is herein dated to 40 332 39 816 intcal13 yrs bp 95 4 confidence interval this terrestrial 14c derived age of the u ym can be used to constrain the chronology of marine records containing the same marker layer this reviewed and integrated tephrostratigraphic framework highlights the pivotal role that distal sedimentary records can play in evaluating the eruptive histories and hazard potential of ulleungdo and changbaishan

age and impacts of the caldera forming aniakchak ii eruption in western alaska the mid holocene eruption of aniakchak volcano aniakchak ii in southwest alaska was among the largest eruptions globally in the last 10 000 years vei 6 despite evidence for possible impacts on global climate the precise age of the eruption is not well constrained and little is known about regional environmental impacts a closely spaced sequence of radiocarbon dates at a peatland site over 1000 km from the volcano show that peat accumulation was greatly reduced with a hiatus of approximately 90 120 yr following tephra deposition during this inferred hiatus no paleoenvironmental data are available but once vegetation returned the flora changed from a cyperaceae dominated assemblage to a poaceae dominated vegetation cover suggesting a drier and or more nutrient rich ecosystem oribatid mites are extremely abundant in the peat at the depth of the ash and show a longer term increasingly wet peat surface across the tephra layer the radiocarbon sample immediately below the tephra gave a date of 1636 1446 cal yr bc suggesting that the eruption might be younger than previously thought our findings suggest that the eruption may have led to a widespread reduction in peatland carbon sequestration and that the impacts on ecosystem functioning were profound and long lasting

environmental change and tephra deposition the strath of kildonan northern scotland pollen analysis tephra counts and geochemical data are presented from the upper 200 cm of a core from a basin mire in the strath of kildonan northern scotland three main tephra concentrations are located and tentatively dated to c 7650bp k1 c 4250 bp k2 and 3250 bp k3 by cross correlation with the regional pollen chronology the identity of the tephra layers is unknown but the peak at 4250 bp may be from the hekla 4 eruption the associated changes in catchment soils and vegetation are highly variable k1 is associated with a major vegetation disturbance but no significant change occurs at k3 k2 is correlated with a large regional rise in pine pollen these results conflict with previous studies on distal impacts of volcanic activity during the holocene and demonstrate the diversity of environmental changes associated with holocene volcanic activity

multi proxy dating the millennium eruption of changbaishan to late 946 ce ranking among the largest volcanic eruptions of the common era ce the millennium eruption of changbaishan produced a widely dispersed tephra layer known as the b tm ash which represents an important tie point for palaeoenvironmental studies in east asia hitherto there has been no consensus on its age with estimates spanning at least the tenth century ce here we identify the cosmogenic radiocarbon signal of 775 ce in a subfossil larch engulfed and killed by pyroclastic currents emplaced during the initial rhyolitic phase of the explosive eruption combined with glaciochemical evidence from greenland this enables us to date the eruption to late 946 ce this secure date rules out the possibility that the millennium eruption contributed to the collapse of the bohai kingdom manchuria korea in 926 ce as has previously been hypothesised further despite the magnitude of the eruption we do not see a consequent cooling signal in tree ring based reconstructions of northern hemisphere summer temperatures a tightly constrained date for the millennium eruption improves the prospect for further investigations of historical sources that may shed light on the eruption s impacts and enhances the value of the b tm ash as a chronostratigraphic marker

marine magnetic anomalies palaeomagnetism and the drift history of gondwanaland abstract the results of palaeomagnetic surveys of mesozoic and tertiary rocks from gondwanaland can be reconciled with the results of modelling the evolution of oceanic floors from analyses of marine magnetic anomalies previous inconsistencies were mainly due to errors apparent in the australian cenozoic palaeomagnetic data an alternative tertiary apparent polar wander path apwp has been constructed from an analysis of all published laterite and weathered profile data palaeomagnetic results for africa antarctica australia india madagascar and south america are compared for rotations corresponding to marine magnetic anomalies 16 22 28 34 and m1 and for fit india has been selected as the reference continent since it provides the most detailed apwp having drifted about 50 of latitude since breakup

late cretaceous polar wander of the pacific plate evidence of a rapid true polar wander event we reexamined the late cretaceous early tertiary apparent polar wander path for the pacific plate using 27 paleomagnetic poles from seamounts dated by 40ar 39ar geochronology the path shows little motion from 120 to 90 million years ago ma northward motion from 79 to 39 ma and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 2 ma the latter phenomenon may represent a rapid polar wander episode 3 to 10 degrees per million years whose timing is not adequately resolved with existing data similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle true polar wander which may have been related to global changes in plate motion large igneous province eruptions and a shift in magnetic field polarity state

the impact of mt mazama tephra deposition on forest vegetation in the central cascades oregon usa the eruption of mt mazama c 7630 yr bp was the largest north american volcanic event during the holocene high resolution pollen and charcoal analyses were used to examine the impact of mt mazama tephra on forest vegetation and possible synergistic interactions with fire activity in the central oregon cascade range we selected four small watersheds on a longitudinal transect north of mt mazama and recovered lake sediment that spanned the period of tephra deposition our sediment records had between 14 and 50 cm of tephra deposited and we analyzed the sediment at centimeter resolution before and after the deposition horizon in each sediment record our analysis shows that nonarboreal pollen percentages and accumulation rates were depressed after mazama tephra deposition recovery to pre tephra deposition rates occurred after approximately 50 100 years arboreal pollen percentage and accumulation rates were less severely impacted suggesting that the mazama tephra deposition disrupted understory communities more significantly than overstory species and that forest communities returned to their pre tephra deposition conditions after approximately 50 100 years fire events in conjunction with the mazama tephra occurred in two of the four sites suggesting that tephra deposition did not create conditions that precipitated a fire event in a consistent way this research reinforces the notion that disturbance events may have cumulative effects on forest vegetation but that the impacts of disturbance events need to be felt by similar constituents of the forest ecosystem to be truly additive

true polar wander and plate driving forces a net torque on the global lithosphere can be exerted both by ridge push and trench pull forces a net ridge push torque is present if there is an asymmetry in the age distribution of seafloor about one or more ocean ridges a net torque due to trench forces is present if the forces per trench length acting on the subducting and overthrust plates are not equal in magnitude the combined net torque on the lithosphere contributed by ridge and trench forces must be balanced by an opposing torque most likely due to basal shear tractions associated with a global rotation of the lithosphere relative to the underlying mantle such a rotation should be identifiable as true polar wander the lack of significant rotation of the lithosphere with respect to the magnetic dipole axis during the cenozoic therefore places a strong constraint on the nature of torques acting on the lithosphere we suggest that the most likely explanation for negligible true polar wander in the cenozoic is that the net ridge and trench torques nearly cancel one another the net ridge and trench torque vectors are in nearly opposite directions and have been so through the cenozoic so the condition of near cancellation amounts to a restriction on the magnitude of the force imbalance along trench boundaries such a near cancellation of net torques and the resulting negligible true polar wander are likely to be consequences of present plate geometries and need not be representative of earlier eras

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

ash from changbaishan millennium eruption recorded in greenland ice implications for determining the eruption s timing and impact major volcanic eruptions can impact on global climate by injecting large quantities of aerosols and ash into the atmosphere that alter the radiative balance and chemical equilibrium of the stratosphere the millennium eruption of tianchi paektu china north korea was one of the largest late holocene eruptions uncertainty about the precise timing of the eruption has hindered the recognition of its climate impact in palaeoclimate and historical records here we report the compelling identification of the eruption s volcanic signal in greenland ice cores through the association of geochemically characterized volcanic glass represented in by bimodal populations that compare with proximal material from the source eruption the eruption most probably occurred in the a d 940s 7 years after the eldgja eruption on iceland we examine the eruption s potential for climate forcing using the sulfate records from the ice cores and conclude that it was unlikely to have had a global or extraregional impact

how a wet tropical rainforest copes with repeated volcanic destruction the holocene period for the province of west new britain papua new guinea is characterised by periodic catastrophic volcanism the region is mantled in dense wet tropical rainforest and has been occupied by people since the pleistocene analyses of peat from two nearby sites within a lowland rainforest environment provide us with a macro level landscape account of the periodic destruction and recovery of the coastal forests during seven periods of volcanic activity in the latter part 2900 yr ago to present of the holocene radiocarbon dating shows the very close correlation of the peat and tephra layers at both sites yet the pollen analysis reveals different vegetation communities these initial results allow us to begin identifying the processes of recovery and to recognise different ecological pressures placed on vegetation at these neighbouring sites evidence of hydrological changes are observed beginning with a marine incursion recorded at garu site 3 1360 14 c yr b p the distinct differences in the vegetation re establishment and community regeneration rates suggest the greater level of disturbance at garu site 1 could be related to the depth of the ashfall although the proximity of a known human settlement may also be a contributing factor of note palynologically we found that the fern spore flora is particularly rich and believe it will be useful for ecological interpretation d 2005 university of washington all rights reserved

paleomagnetic and plate tectonic constraints on the movement of tibet abstract the paleomagnetic results from tibet north of the yarlung zang bo suture zone show that tibet was at about 15 20 n in middle cretaceous time it then moved south down to 7 10 n in the late cretaceous paleogene the oceanic crust of the xigaze ophiolites was magnetized at 13 n but thereafter migrated further south this movement is compared with the relative movement of india and asia as deduced from magnetic anomalies and paleomagnetism experimental models on deformation help us to explain how tibet moved during the late cretaceous under the constraint of the africa arabia indenter and during the upper tertiary under the constraint of the indian indenter

volcanic impacts on peatland microbial communities a tephropalaeoecological hypothesis test abstract volcanic eruptions affect peatlands around the world depositing volcanic ash tephra and a variety of chemicals including compounds of sulphur these volcanic impacts may be important for many reasons in particular sulphur deposition has been shown to suppress peatland methane flux potentially reinforcing climatic cooling experiments have shown that sulphur deposition also forces changes in testate amoeba communities potentially relating to the reduced methane flux large volcanic eruptions in regions with extensive peatlands are relatively rare so it is difficult to assess the extent to which volcanic eruptions affect peatland microbial communities palaeoecological analyses across tephra layers provide a means to resolve this uncertainty in this study testate amoebae were analysed across multiple monoliths from a peatland in southern alaska containing two tephras probably representing the 1883 eruption of augustine volcano and a 20th century eruption of redoubt volcano results showed relatively distinct and often statistically significant changes in testate amoeba community coincident with tephra layers which largely matched the response found in experimental studies of sulphur deposition the results suggest volcanic impacts on peatland microbial communities which might relate to changes in methane flux

the impact of high tephra loading on late holocene carbon accumulation and vegetation succession in peatland communities abstract peatlands are major terrestrial stores of carbon c of importance to the global climate system recent studies have made progress in understanding the climatic controls on the c cycle however important interactions between volcanic deposition and peatland c stores remain to be addressed this study uses a 3000 year peatland record from northern japan to examine the interactions between carbon accumulation vegetation community succession and volcanic ash deposition plant macrofossil and testate amoebae records are presented alongside records of total organic carbon nitrogen and phosphorous age depth models are developed using a bayesian approach with seven ams radiocarbon dates and two identified historical tephras from baitoushan ad 969 981 cal bp and hokkaido komagatake ad 1640 310 cal bp volcanoes results show that moderate to high tephra loading can shift peatland plant communities from sphagnum to monocotyledon domination this vegetation change is associated with increased peat humification and reduced carbon accumulation where tephra deposition and reworking has occurred the apparent rate of carbon accumulation can be halved while high tephra loading of the mire surface is sustained sphagnum species vary in their tolerance to tephra deposition after each ash fall sphagnum magellanicum disappeared from the plant macrofossil record whereas sphagnum papillosum showed apparent continuity of development through the 1856 94 cal bp ko c1 tephra high rates of carbon accumulation peaking at 100 g m 2 yr 1 2 3 times faster than the average for northern peatlands were recorded in the sphagnum communities that established after the cessation of tephra deposition and reworking from the ad 969 baitoushan ash fall b tm tephra this peak in c accumulation was coincident with a radical shift in mire nutrient cycling most probably caused by the interaction of s magellanicum with leachates from the underlying tephras the phase of high c accumulation continued for over 300 years offsetting the initial negative impact of the b tm tephra on peatland c accumulation these results suggest that management for ash tolerant sphagnum species could be a highly effective strategy for minimising volcanic disruption to peatland carbon accumulation the study also shows that consideration of volcanic impacts on peatlands is essential for development of more realistic terrestrial carbon balance models in volcanically active regions

late mesozoic and cenozoic palaeomagnetism of australia ii implications for geomagnetism and true polar wander summary new palaeomagnetic results from australia indicate that throughout the cenozoic era the continent lay further south than suggested by hot spot data moreover while hot spots give a uniform rate of drift during most of the cenozoic the drift rate obtained from apparent polar wander varies considerably the discrepancies between the palaeomagnetic and hot spot results are analysed by comparing the australian data with those of europe and the central pacific the analysis suggests that the discrepancies are due to 1 departures of the earth s magnetic field from the geocentric axial dipole model and 2 either true polar wander or a non axial inclined dipole component it is found that since the mid tertiary the dominant non dipole component has been a quadrupole and that during this period both the quadrupolar field and the true polar displacement non axial dipole component decreased progressively during the quaternary and also at the earliest tertiary the non dipole components appear to have been moderate or small the comparison of data sets demonstrates that considerable errors may be incurred when cenozoic and presumably earlier poles from one geographic region are used to derive those of another widely separated region the results also imply that absolute plate velocities estimated from palaeomagnetic data can contain substantial errors and that hot spot data may need significant adjustments for true polar wander to yield correct palaeolatitudes finally the new early tertiary pole for australia is used in conjunction with updated early tertiary poles from other lithospheric plates to reapply the mckenzie test for true polar wander the results indicate a small true polar displacement since the beginning of the tertiary the amount and direction of the displacement however differ from those generally obtained from hot spot data

millennium scale volcanic impact on a superhumid and pristine ecosystem ecosystems damaged by distal volcanic ash and sulfur deposition usually recover within decades however sediment stalagmite and pollen records from the southernmost andes indicate a 2000 yr impact on forest and aquatic ecosystems after deposition of a thin tephra layer so 2 released from altering pumice produced intense soil and lake acidification in a 150 000 km 2 area acidification led to nutrient leaching and affected soil microorganisms causing plant decay and increased soil erosion in an area larger than 8000 km 2 we conclude that weakly buffered soils in humid environments are extremely vulnerable to volcanic and anthropogenic acidification causing long lasting ecosystem damage and perturbations of paleoclimate proxy records

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

vegetation history and the impact of tephra deposition during 7000 years based on pollen and tephra analysis of a barasantou bog sediment core eastern hokkaido northern japan abstract to estimate the impact of a volcanic eruption on vegetation we conducted a field survey and collected peaty sediments and made pollen and tephra analysis from barasantou bog in eastern hokkaido northern japan using an electron probe micro analyzer we identified five tephra layers in the core ma g 7 6 cal ka bp ma e 5 5 cal ka bp ma d 4 0 cal ka bp ta c 2 5 cal ka bp and ko c2 ad 1694 when the peat began to accumulate again after deposition of the ma g tephra the groundwater level of this area was very low and alnus poaceae thalictrum artemisia and ferns dominated gradually the groundwater level increased and the condition of this wetland approached the present conditions about 5 0 to 4 5 cal ka bp pinaceae conifers increased 2 5 cal ka bp it is presumed that picea glehnii and abiesspecies formed wetland forests myrica gale and alnus increased again and grew to the edge of the wetland at the same time quercus subgen lepidobalanus decreased and betula increased this trend might be due to the influence of human activity the wetland vegetation changed from thalictrum and artemisia to sanguisorba and then from sanguisorba to menyanthes after the deposition of ma g tephra or after 2 5 ka bp it was thought that the wetland changed from low moor to middle or high moor a tephra layer tens of cm thick is not expected to affect pollen composition and may have no clear impact on the surrounding vegetation sediment with a thickness of a few cm requires a deposition time of approximately 10 years pollen analysis results are on the scale of the average vegetation change during that period we conclude that short term vegetation changes were not detected

large scale motion between pacific and atlantic hotspots studies of true polar wander tpw the rotation of the solid earth with respect to the spin axis1 have suggested that there has been 10 15 of relative motion over the past 130 myr refs 2 4 in such studies the orientation of the spin axis is recovered from continental palaeomagnetic poles corrected for relative plate motions and compared with a deep mantle reference frame defined by hotspot locations but deducing relative plate motions becomes increasingly difficult for older mesozoic time periods hindering tests of tpw on timescales comparable to those of large scale mantle convection moreover the assumption of hotspot fixity is controversial5 6 we examine here a more direct approach7 8 using palaeolatitudes derived from pacific guyots contrary to predictions from tpw models these data suggest only minor latitudinal shifts of pacific hotspots during the cretaceous period instead of tpw relative motion between the atlantic and pacific hotspot groups9 is required at a velocity of approximately 30 mm yr 1 more than 50 larger than previously proposed5

distal volcanic impacts on peatlands palaeoecological evidence from alaska abstract despite the fact that volcanic ash tephra layers are found preserved in peat deposits around the world comparatively little research has investigated the impacts of distal volcanic emissions on peatlands this study investigates the impacts of several late holocene volcanic eruptions on five peatlands in southern alaska using a palaeoecological approach testate amoebae analysis peat humification analysis and a basic analysis of plant macrofossil components were applied across 11 tephra layers changes in macrofossil and testate amoebae assemblages occur across several of the tephra layers the humification results were considered unreliable because of a methodological problem a finding which may have implications for other studies using this technique redundancy analyses on testate amoebae data show statistically significant changes associated with two tephras the most likely causes of the impacts are volcanic gases acidic precipitation or tephra derived leachates the finding that some tephras are associated with impacts whereas others are not may relate to the season of the eruption or meteorological conditions at the time of ash fall these results suggest the sensitivity of peatlands and peatland microbial communities to distal volcanic products and imply that changes in key palaeoclimatic proxies may be caused by a mechanism independent of climate change implications of the results for peat based palaeoclimatic studies are discussed as are possible directions for future research

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

high precision ultra distal holocene tephrochronology in north america far travelled volcanic ashes tephras from holocene eruptions in alaska and the pacific northwest have been traced to the easternmost extent of north america providing the basis for a new high precision geochronological framework throughout the continent through tephrochronology the dating and correlation of tephra isochrons in sedimentary records the reported isochrons are geochemically distinct with seven correlated to documented sources in alaska and the cascades including the mazama ash from oregon w7600 years old and the eastern lobe of the white river ash from alaska w1150 years old these findings mark the beginning of a tephrochronological framework of enhanced precision across north america with applications in palaeoclimate surface process and archaeological studies the particle travel distances involved up to w7000 km also demonstrate the potential for continent wide or trans atlantic socio economic disruption from similar future eruptions

the earth as a planet paradigms and paradoxes the independent growth of the various branches of the earth sciences in the past two decades has led to a divergence of geophysical geochemical geological and planetological models for the composition and evolution of a terrestrial planet evidence for differentiation and volcanism on small planets and a magma ocean on the moon contrasts with hypotheses for a mostly primitive still undifferentiated and homogeneous terrestrial mantle in comparison with the moon the earth has an extraordinarily thin crust the geoid which should reflect convection in the mantle is apparently unrelated to the current distribution of continents and oceanic ridges if the earth is deformable the whole mantle should wander relative to the axis of rotation but the implications of this are seldom discussed the proposal of a mantle rich in olivine violates expectations based on evidence from extraterrestrial sources these and other paradoxes force a reexamination of some long held assumptions

the paleomagnetic field from equatorial deep sea sediments axial symmetry and polarity asymmetry paleomagnetic data from 89 equatorial deep sea sediment cores indicate that the configuration of the time averaged geomagnetic field depends strongly on polarity state but that it remains within 1 degree of axial symmery throughout the pliocene and pleistocene last 5 million years the relative magnitude of the nondipole field was greater by almost a factor of 2 during reverse than during normal polarity intervals these results thus support earlier suggestions that there may be a standing nonreversing component of the geomagnetic dynamo

late quaternary tephrostratigraphy of southern chile and argentina abstract the southern and austral volcanic zones of the andes comprise 74 volcanic centres with known post glacial activity at least 21 of these have had one or more large explosive eruptions in the late quaternary dispersing tephra over vast areas these tephra layers therefore have great potential as tephrochronological marker horizons in palaeoenvironmental studies in southern chile and argentina a region that is particularly useful to study climate dynamics of the southern hemisphere however to date tephrochronology has rarely been fully utilised in this region as a correlation and dating tool here we review the existing post glacial tephrostratigraphic record of the southern and austral volcanic zones and compile a database of known occurrences of tephra from these volcanoes in ice and lacustrine marine peat and cave sediment records we address the inconsistencies in and revisions of the tephrostratigraphies presented in prior literature and discuss the challenges in correlating tephras and the limitations of the tephrostratigraphic record in this area this study highlights the many gaps that still exist in our knowledge of the eruptive histories of these volcanoes but also reveals the largely under utilised potential of tephra as a correlation tool in this region this is exemplified by the severe lack of adequate geochemical analysis of tephra layers preserved in many lacustrine and peat sediment sections which are particularly important tephrostratigraphic records in southern chile and argentina due to the paucity of surface preservation

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

long term mobilisation of chemical elements in tephra rich peat ne iceland this paper presents geochemical profiles of a tephra bearing minerotrophic peat column from ne iceland obtained using various elemental analyses of the solid phase and the pore water the influence of tephra grain size thickness and composition of each tephra on the peat geochemistry was investigated interpretations are supported by a statistical approach in particular by autocorrelation and by microscopy observations minerotrophic peat geochemistry may be strongly dependent upon post depositional mobilization and possible leaching of elements as demonstrated by fe and trace metal concentration profiles chemical elements and more specifically potentially harmful metals can be slowly leached out of volcanic falls during their weathering and re accumulate downwards it is emphasised that a tephra deposit can act as an active geochemical barrier blocking downward elemental movements and leading to the formation of enriched layers in this study the formation of poorly amorphous fe phases above the hekla 3 tephra is shown these poorly crystalline fe phases scavenged ni

effects of volcanic activity on mire development case studies from hokkaido northern japan field stratigraphy and macrofossil composition of four mires in hokkaido northern japan were investigated to test whether tephra deposition induces long term vegetation shifts macroscopic tephras of known origin ranging from 0 5 to 25 cm in thickness were found in two or more of the study sites allowing correlation of the stratigraphic development of the mires major peat types and position of macroscopic mineral layers were determined in the field and selected cores were analysed by quantitative macrofossil analysis some macrofossil groups and the content of unidentifiable organic matter showed changes in the vicinity of tephra layers but they did not indicate a clear response pattern of the vegetation or the decomposition dynamics of plant material mosses were expected to be most strongly affected by tephra deposition but even in cases where tephra fell on sphagnum dominated vegetation no fundamental shifts to new plant communities were found this suggests that disturbance by widespread tep

true polar wander an analysis of cenozoic and mesozoic paleomagnetic poles a true polar wander path is presented for the past 180 ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data a relative displacement of 22 10 is observed the motion between the two reference frames has not been smooth episodes of rapid motion on the order of 80 100 mm yr include the present 65 50 ma 115 85 ma and 180 160 ma periods of slow or little polar motion less than 20 mm yr are observed at 50 5 ma and 160 115 ma the effect of plate motions is removed using morgan s plate motion model a19 this model is tied to a hotspot reference frame assumed to be fixed in the mantle the rotated paleopoles are grouped by ages and then averaged they are found to cluster about points distinct from the north pole as defined by the hotspots these points trace out a path of motion between the rotation axis and the hotspots or mantle this is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle paleomagnetic poles of the past 7 m y are examined independently this eliminates a dependence on the fixed hotspot hypothesis since plate motions are essentially negligible for this time span there is a deviation of 5 2 between the present geographic north pole and the average of the paleopoles at 5 ma this is too great to attribute to plate motion interpreted as true polar wander the rate of this polar motion is about 1 m y astronomical measurements for the past 80 years have recorded actual motion of the earth s rotational axis up to 1 m y in the direction of eastern canada this agrees in rate with paleomagnetic polar motion of the past 7 m y and in both rate and direction for the past 2 m y

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

magnitude and significance of carbon burial in lakes reservoirs and peatlands globally lakes are currently accumulating organic carbon oc at an estimated annual rate of about 42 tg yr 1 most of the oc in all but the most oligotrophic of these lakes is autochthonous produced by primary production in the lakes the sediments of reservoirs accumulate an additional 160 tg annually and peatlands contribute 96 tg annually these three carbon pools collectively cover less than 2 of the earth9s surface and constitute a carbon sink of about 300 tg yr 1 although the oceans cover 71 of the earth9s surface they accumulate oc at a rate of only about 100 tg yr 1

paleomagnetism of abbott seamount and implications for the latitudinal drift of the hawaiian hot spot the magnetic anomaly of abbott seamount located near the hawaiian emperor bend has been analyzed for its paleomagnetic information short wavelength noise components of the anomaly assumed to result from small magnetization inhomogeneities or reversals in the upper layers of the volcano were filtered out by upward continuation the polarity of the bulk of the seamount is reversed and its virtual geomagnetic pole is located at 75 5 n 4 6 e this pole lies close to pacific paleomagnetic poles of late eocene age and is consistent with the age of the edifice to be expected from its position in the hawaiian chain the paleolatitude of the seamount 17 5 n 4 4 is not significantly different from the present latitude of the hawaiian hot spot approximately 19 5 n this finding implies that the hawaiian hot spot and by inference other hot spots has remained relatively fixed with respect to the spin axis since the eocene however other paleolatitudes from the hawaiian emperor chain suggest that systematic variation of the hot spot s latitude may have occurred comparison of the hawaiian emperor paleolatitude data with paleomagnetic data from central pacific piston cores and with north pacific paleoequator data from deep sea drilling project cores indicates that the apparent latitude drift since the eocene has been largely the result of long term nondipole components in the geomagnetic field rather than relative motion between the hot spot and the spin axis conversely the variation that occurred prior to the eocene appears to contain a significant amount of such motion coupled with an indeterminate amount of nondipole effect

holocene tephrostratigraphic framework and monsoon evolution of east asia key tephra beds for synchronising palaeoclimate records abstract in east asia our understanding of holocene climate change forcing mechanisms and propagation require the precise chronological control of palaeoclimate records to allow robust integration of data sets the existing chronologies predominantly based on 14c method however are not sufficient to constrain key questions about abrupt climate shifts that occur within a century in the transitions between states widely dispersed tephra layers allow precise dating and synchronisation of sedimentary archives providing a chronological framework for integrating records especially where the visible tephra record is complemented by the addition of cryptotephra non visible ash despite significant tephra studies in this region however a comprehensive holocene tephra framework is not available to address this issue we carry out a thorough review on holocene tephra investigations undertaken in japan using widespread tephra beds we present an integrated tephra framework and suggest the way forward for establishing this as a wider approach for east asia the framework is based on twenty two ash layers that are mainly from japan and to a lesser extent china n korea s korea and russia each tephra is assessed in terms of chronology geochemistry and distribution the framework is compared with high resolution palaeoclimate records from east asia using this we demonstrate regional variations in monsoon evolution and more importantly the potential of tephra isochrons in constraining such variations given the scarce identification of tephra layers in those well resolved palaeoclimate records we advocate a more systematic employment of the cryptotephra method which would potentially lead to a significant advance in east asian tephrochronology and the correlation of palaeoclimate archives in the region

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

a sensitive high resolution record of late holocene climatic change from a raised bog in northern england proxy climatic data in the form of plant macrofossils have been analysed from a 5 m core from bolton fell moss cumbria uk detailed analysis of peat from the upper 50 cm of this core is used to demonstrate a strong correlation between changes in the relative proportion of taxa and known climatic changes over the last 1000 years the record of changes in bog vegetation contained within the peat profile is used to reconstruct changes in bog surface wetness for the latter half of the holocene as bog surface wetness is directly controlled by the prevailing climatic conditions this reconstruction can be viewed as a proxy climate record twelve radiocarbon age estimates on the 5m core suggest that between 50 and 500 cm peat accumulated at a relatively constant rate of 12 4 yr cm 1 the regular sampling intervals thus provide a time series of past bog surface wetness spectral analyses of this series indicates that wetness changes are cyclic with a ca 800 year periodicity

latitudinal shift of pacific hotspots during the late cretaceous and early tertiary two frames of reference the geomagnetic field and hotspots have been widely used to measure the past motions and interactions of lithospheric plates several studies have found that these reference frames have undergone as much as 19 22 of relative motion often called true polar wander tpw in the past 200 myr1 4 moreover it also appears that this offset did not accumulate at a uniform rate some evidence suggests that spurts of tpw can occur at rates as high as 1 2 per myr3 5 rates even an order of magnitude less could pose serious problems for tectonic studies based on the hotspot reference frame because velocities of this magnitude cannot be considered insignificant compared to those of the plates unfortunately the imprecision of most palaeomagnetic data is such that large error bounds are common for many estimates of tpw rates in this report we present a record of tpw derived from pacific palaeomagnetic data that are relatively well constrained in age and palaeolatitude they show that offset between the two reference frames accumulated at 0 3 per myr 1 from late cretaceous to early tertiary

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

assessing the impact of icelandic volcanism on vegetation systems in the north of ireland in the fifth and sixth millennia bc the lairg tephras comprise two of the three most abundantly represented tephra layers in irish holocene peat sequences the impact of these tephras on the vegetation at two lowland raised bogs in northern ireland is investigated through high resolution pollen percentage and concentration studies the palynological response of mire and dryland taxa to the tephra falls is assessed no significant palynological response to either tephra event is recorded

ecology and paleoenvironmental application of testate amoebae in peatlands of the high elevation colombian paramo we investigated the ecology and paleoecology of testate amoebae in peatlands of the colombian paramo to assess the use of testate amoebae as paleoenvironmental indicators objectives were to 1 identify environmental controls on testate amoebae 2 develop transfer functions for paleoenvironmental inference and 3 examine testate amoebae in a holocene peat core and compare our findings with other proxy records results from 96 modern samples indicate that testate amoebae are sensitive to ph and surface moisture and cross validation of transfer functions indicates potential for paleoenvironmental applications testate amoebae from the triunfo peatland in the central cordillera provided a proxy record of ph and water table depth for the late holocene and inferred changes were correlated with peat c n measurements during most of the record comparison with a lake level reconstruction suggests that at least the major testate amoeba inferred changes were driven by climate our work indicates that testate amoebae are useful paleoenvironmental indicators in high elevation tropical peatlands

simulating the impacts of distal volcanic products upon peatlands in northern britain an experimental study on the moss of achnacree scotland the potential impact of prehistoric volcanism on distal areas has been the subject of some discussion in both ecology and archaeology an experimental approach is used here to examine the potential effects of distant volcanism on a peatland environment simulations of different tephra and acidity loading designed to approximately replicate the prehistoric hekla 4 ashfall were monitored over 2 years impacts on the peatland ecosystem were assessed by qualitative observations of plant health and abundance semi quantitative observations of flowering measurements of peat ph humification and testate amoebae community composition plots with higher acid loading showed immediate and lasting impacts on plants although other treated plots were less affected changes in testate amoebae and peat humification were inconsistent both within and between plots the experiment demonstrates the potentially severe effects of high acid loading on peatland plants although some responses remain unclear

holocene comment and reply hekla 4 ash the pine decline in northern ireland and the effective use of tephra isochrones a comment on hall pilcher and mccormac in response to a paper by blackford et al 1992 which showed a coincidence between the deposition of hekla 4 tephra and a decline of pinus pollen in two northern scottish sites hall et al 1994 investigated the tephra pine pollen relationship at two sites in northern ireland we argue that their sites are inappropriate for such comparison because pine was effectively absent from co antrim long before the deposition of hekla 4 the opportunity is taken to make some additional points concerning the irish data and the wider use of tephra isochrones

a mid to late holocene cryptotephra framework from eastern north america abstract holocene cryptotephras of alaskan and pacific northwestern origin have recently been detected ca 7000 km away on the east coast of north america this study extends the emerging north american tephrochronological framework by geochemically characterising seventeen cryptotephra layers from four newly explored peatlands all detected tephras were deposited during the late holocene with no horizons present in the peat between ca 3000 5000 years ago the prevalence of the alaskan white river ash eastern lobe ad 847 1 is confirmed across the eastern seaboard from newfoundland to maine and a regional depositional pattern from mount st helens set w ad 1479 1482 is presented the first occurrences of four additional cryptotephras in eastern north america are described three of which may originate from source regions in mexico kamchatka russia and hokkaido japan the possibility of such tephras reaching eastern north america presents the opportunity to link palaeo archives from the tropics and eastern asia with those from the western atlantic seaboard aiding inter regional comparisons of proxy climatic records

cenozoic global plate motions cenozoic global plate motions relative to the hot spots are investigated and compared to plate motions in a mean lithosphere reference frame plate motions were analyzed over six time intervals divided by ages 10 25 43 48 and 56 ma chosen as much as possible to coincide with key plate reorganizations alternative motion circuits and rotational parameters were considered and evaluated with paleomagnetic data from the pacific and north american plates the circuit found to be in best agreement with the paleomagnetic data is one in which the hot spots in the atlantic region are assumed to be fixed relative to the hot spots in the pacific region throughout the cenozoic the hot spot and mean lithosphere reference frames have been in continual slow relative motion the rate of motion is nonuniform however most of the motion having occurred during the middle cenozoic the net cenozoic rotation of the lithosphere relative to the hot spots is described by a right handed rotation of 7 about a euler pole at 46 s 87 e which yields a 5 displacement of the north poles of the two reference frames this motion is small enough that inferences drawn about plate speeds in one reference frame should be valid in the other analysis of the global motions resulting from our preferred model showed that many characteristics of current plate motions have persisted throughout the cenozoic plate speeds correlate with latitude plates moving faster near the equator than near the poles throughout the cenozoic as at present continental plates except for the indian plate moved slower than oceanic plates throughout the cenozoic even the structure of the velocity fields as revealed in a contour of root mean square velocities in equatorial bands persists throughout the cenozoic the migration of the paleomagnetic axis over time is also compared to the hot spot and mean lithosphere reference frames the paleomagnetic axis has shifted 5 10 relative to the hot spot frame and a lesser amount relative to the mean lithosphere frame

hekla 4 ash the pine decline in northern ireland and the effective use of tephra isochrones a reply to edwards dugmore buckland blackford and cook

tephrostratigraphy of changbaishan volcano northeast china since the mid holocene abstract a detailed tephrostratigraphy of an active volcano is essential for evaluating its eruptive history forecasting future eruptions and correlation with distal tephra records changbaishan volcano is known for its millennium eruption me ad 940s vei 7 and the me tephra has been detected in greenland ice cores 9000 km from the vent however the pre millennium pre me and post millennium post me eruptions are still poorly characterized in this study we present a detailed late holocene eruptive sequence of changbaishan volcano based on single glass shard compositions from tephra samples collected from around the caldera rim and flanks tephra ages are constrained by optically stimulated luminescence osl and ams 14c dates tephra from the mid holocene pre me eruption can be divided into two pyroclastic fall subunits and it cannot be correlated with any known changbaishan sourced tephra recorded in the japan sea based on major element composition of glass shards such as the b j baegdusan japan basin and b v baegdusan vladivostok oki tephras me pyroclastic fall deposits from the caldera rims and volcanic flanks can be correlated to the juvenile pumice lapilli or blocks within the pyroclastic density current pdc deposits deposited in the valleys around the volcano based on glass shard compositions our results indicate that the glass shard compositions of proximal me tephra are more varied than previously thought and can be correlated with distal me tephra in addition widely dispersed mafic scoria was ejected by the me plinian column and deposited on the western and southern summits and the eastern flank of the volcano data for glass from post me eruptions such as the historically documented ad 1403 ad 1668 and ad 1702 eruptions are reported here for the first time except for the me other holocene eruptions including pre me and post me eruptions had the potential to form widely distributed tephra layers around northeast asia and our dataset provides a proximal reference for tephra and cryptotephra studies in surrounding areas

paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic abstract we first re examined the apparent polar wander curves for stable eurasia and africa since the triassic these curves were then combined together with curves of north and south america according to the kinematics of the atlantic ocean and a synthetic polar wander curve was given then most of the paleomagnetic results from the tethys mobile belt from the atlantic to the pamirs were analysed several groups of plates microplates and blocks can be seen first relatively stable regions like maghreb and sicily which have not moved much then we have a group formed by iberia sardinia italy and to a lesser extent corsica and the western and central alps for these blocks movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of africa but they are sometimes stronger to the east a major change takes place the north of the aegean sea and the ionian zone are clockwise rotated and these rotations are recent oligocene miocene for the first part pliocene to the present for the second part a major problem arises in turkey caucasus and iran paleomagnetic results indicate a position far to the south of eurasia and at the same time geological evidence is in favour of a position close to eurasia we discuss these discrepancies

icelandic volcanic ash and the mid holocene scots pine pinus sylvestris pollen decline in northern scotland a volcanic ash layer in peat from northern scotland has been identified and coincides exactly with an abrupt decline in pinus sylvestris l scots pine pollen frequencies this provides an isochrone time equivalent marker horizon with which to investigate the timing of the holocene pine decline furthermore two possible causes of the southward shift of the range of pinus in scotland c 4000 bp are suggested a direct effect of acid pollution by chemicals produced by the eruption of hekla h 4 or a volcanically induced climatic perturbation these possibilities have wider implications for the influence of volcanism on postglacial environmental change

new evidence for the presence of changbaishan millennium eruption ash in the longgang volcanic field northeast china abstract the changbaishan millennium eruption ad 940s produced a widely distributed tephra layer around northeast asia this tephra layer serves as a marker bed in greenland ice cores and in marine lake archeological and tsunami sediments in japan and the surrounding region however little attention has been paid to the widespread sediments west of changbaishan volcano here we present new stratigraphic geochemical varve chronology and 14c geochronological data from the varved sediments in lake sihailongwan longgang volcanic field northeast china extending the westerly margin of this eruption the distinctive geochemical characteristic of volcanic glass ranging from trachyte to rhyolite similar to those of proximal and distal tephra confirmed the occurrence of changbaishan millennium eruption ash in the lake illustrating the westward dispersal fan of the ash deposits the position of the peak concentration of glass shards of this tephra was dated to 953 37 ad by varve chronology and the radiocarbon samples immediately above this tephra gave a date of 940 1020 ad overlapping the most recent ages for this eruption the occurrence of changbaishan millennium eruption ash in this lake enables a direct and precise synchronization with other high resolution archives in northeast asia such as maar lakes and peat and marine sediments thus providing an isochronous marker for a range of sedimentary contexts

an inter laboratory comparison of the electron probe microanalysis of glass geochemistry abstract tephrochronology commonly relies upon grain discrete analysis of glass shards to reveal the subtle geochemical differences between tephras from past explosive volcanic eruptions the use of electron probe microanalysis for this purpose is widely accepted by tephrochronologists it is widely recognised that both precision and accuracy must be maximised and that rigorous standardisation procedures must be followed in this paper the performances of seven northern european electron microprobe centres are compared using a geochemically homogeneous obsidian secondary standard the results reveal that a number of tephra oriented probe centres could benefit from exercises similar to the programme outlined within in particular it is evident that a number of these centres undertaking tephrochronological investigations have generated geochemical data that may be inadequate in terms of distinguishing between tephras inaccuracy in the determination of sodium is seen to be the main difficulty in recent years the application of distal tephrochronology to stratigraphical problems in the northern european quaternary record has become increasingly common geochemical standardisation is essential at an early stage in the development of a regional tephrochronological framework and we hope exercises such as presented here will encourage the production of reliable tephra geochemical data

true polar wander during the middle paleozoic true polar wander would be recognized paleomagnetically as identical apparent polar wander paths for all surface elements the apparent polar wander paths for the late ordovician late devonian interval for laurentia baltica and gondwana have nearly identical looping shapes that can be brought into superposition the paths of these continents are well documented but even the less well known paths of south china and siberia reveal similar lengths the resulting reconstruction places the northern andean margin of south america opposite the appalachian margin of laurentia with baltica and laurentia adjoined in the fit of bullard and colleagues siberia and south china would be to the north of africa if their paleopoles are taken at face value for middle paleozoic time there is of course no information about oceanic domains butnit is interesting that all continental elements insofar as is known appear to have similar apparent polar wander tracks that can be plausibly superimposed without causing overlap in the positions of the continents albeit speculatively because of the lack of information about the oceanic elements it is suggested in this study that true polar wander may have occurred with a cumulative magnitude of about 75 during a 75 ma interval and may have been of greater magnitude than the apparent polar wander due to relative motions during the middle paleozoic this middle paleozoic rate of true polar wander appears to have been an order of magnitude greater than the average rate during the late mesozoic and tertiary

identification of the changbaishan millennium b tm eruption deposit in the lake suigetsu sg06 sedimentary archive japan synchronisation of hemispheric wide palaeoclimate archives the b tm tephra dispersed during the highly explosive changbaishan millennium eruption ca 940 950 ce and a key marker layer within the greenland ice cores has now been identified in the lake suigetsu sg06 sedimentary sequence central japan the major element geochemistry of the volcanic glasses within this tephra layer are compared to a new glass dataset from the distal type locality tomakomai port hokkaido and other published millennium eruption b tm deposits to verify this correlation the discovery of the b tm tephra in the lake suigetsu record provides to date the most southerly identification of this ash and crucially the first direct tie point between this high resolution mid latitude palaeoclimate archive and the greenland ice cores these findings present significant encouragement for on going research into the tephrostratigraphy of east asia focusing on the identification of widely dispersed tephra layers which can facilitate the synchronisation of disparate palaeoclimate archives and thus enable the assessment of spatio temporal variations in past climatic change

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

plate motions and the geomagnetic field ii jurassic to tertiary summary the analysis of the time averaged geomagnetic field is extended back to 200ma palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region these were returned with their corresponding sampling sites to their locations at the estimated dates of magnetization in a fixed hotspots framework the corrected results were then grouped into 20ma windows at intervals of 10ma representing the past 100ma and 40ma windows at 30ma intervals for the more sparse 100 200ma data global means and fisher statistics were calculated for each window having included the axial quadrupole in the calculation the value of this coefficient which gave the maximum value for the fisher precision parameter tightest grouping of poles was taken as representative of each interval the results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the cenozoic this is equivalent to a northward offset axial dipole field far sided effect during the late cretaceous this component appears to have changed sign with respect to the dipole negative values seem to have obtained throughout the cretaceous long normal polarity interval corresponding to a southward offset dipole near sided effect the data distribution is inadequate for the resolution of the quadrupole at earlier times and zero values cannot be discounted little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 ma the global mean polar path curving around the predicted fixed hotspots pole at a distance of typically 5 latitude with little sign of rapid tertiary polar wander as implied by studies of pacific data alone between 100 and 200 ma however there is a clear difference between the two reference frames amounting to 17 19 in the jurassic this may reflect motion of the mantle relative to the geomagnetic axis but may also include errors due to inaccurate determination of hotspot tracks and inter hotspot motion

icelandic volcanic ash and the mid holocene scots pine pinus sylvestris decline in the north of ireland no correlation tephra linked pollen and dendrochronological studies were performed on mid holocene subfossil peats and pines and two lowland raised bogs in the north of ireland the results demonstrated that at these sites contrary to recent findings in the north of scotland there was no temporal link between the decline in pine pollen and tephra from the eruption of the icelandic volcano hekla h4

bed load at low shields stress on arbitrarily sloping beds failure of the bagnold hypothesis 1 the bagnold hypothesis has been a major tool in the development of mechanistic models of bed load transport according to the hypothesis a necessary constraint for the maintenance of equilibrium bed load transport is that the fluid shear stress at the bed must be reduced to the critical or threshold value associated with incipient motion of grains the constraint determines a functional form for the areal concentration of bed load grains as a function of flow parameters it is shown here however that the bagnold hypothesis breaks down when applied to equilibrium bed load transport on beds with transverse slopes above a relatively modest value that is well below the angle of repose in particular under such conditions it is shown that no areal concentration no matter how large is sufficient to reduce the fluid shear stress at the bed to the critical value this failure motivates the abandonment of the bagnold constraint even for nearly horizontal beds the framework presented here however provides a natural basis for an entrainment based model of sediment transport that neither satisfies nor suffers from the drawbacks of the bagnold constraint

quantitative palaeoclimate estimates for early miocene southern new zealand evidence from foulden maar abstract a rich and diverse plant macrofossil assemblage from an earliest miocene diatomite deposit in southern new zealand provides a detailed record of mid latitude southern hemisphere terrestrial climate in an oceanic setting quantitative palaeoclimate estimates for temperature and precipitation variables were obtained using the climate leaf analysis multivariate program clamp and the bioclimatic analysis approach in addition the surface area of fossil podocarpus travisiae leaves preserved at the site was used as a proxy for mean annual temperature clamp and bioclimatic analysis both estimated mean annual temperatures at 18 5 19 5 c with a moderate seasonal temperature range between summer and winter of 10 c and leaf size in podocarpus suggested a mat of about 21 c precipitation rates were high at around 1700 2000 mm per year clamp analysis suggested a 600 mm difference between the dry and wet season whereas bioclimatic analysis indicated a 200 250 mm difference between summer and winter when compared to clamp analysis on modern vegetation on subtropical oceanic islands it would seem that clamp overestimates seasonal precipitation differences

incipient motion of sand particles on side slopes several criteria have been theoretically proposed for the critical shear stress of noncohesive sediment on side slopes however no laboratory work has ever been done on this problem a theoretical study of the incipient motion of sediment particles on side slopes is presented including the effects of viscous sublayer turbulence and lift force as a special case of level beds the theory describes the experimental values reported by several workers a specially designed rotary wind tunnel permits the evaluation of the critical shear stress on side slopes and the experiment supports the theory these works reveal that the lift force can be neglected in estimating the critical shear stress using the proposed theory if the averaged experimental values are concerned but inclusion of the lift force in the theory is recommended for safety and for describing the lower limit of the possibility of incipient motion