a new indicator of movement direction during orogenesis measurement technique and application to the alps the orientation of axes of sigmoidal staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism a simple technique is presented in detail to find this axis in 3d by radially sectioning a horizontal slab cut from an oriented sample when viewed from one direction the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis this technique is independent of the model adopted for the formation of sigmoidal or spiral shaped inclusion trails and can be used to evaluate the mechanism by which they form it can also be used to evaluate theories of folding and orogenesis measurements of spiral and sigmoid axes in garnet porphyroblasts from the european alps show that they reflect the movement of the african plate relative to europe better than linear indicators preserved within the same rocks

molar gas ratios of air entrapped in ice a new tool to determine the origin of relict massive ground ice bodies in permafrost abstract the molar ratios of atmospheric gases change during dissolution in water due to differences in their relative solubilities we exploited this characteristic to develop a tool to clarify the origin of ice formations in permafrost regions extracted from ice molar gas ratios can distinguish buried glacier ice from intrasedimental ground ice formed by freezing groundwaters an extraction line was built to isolate gases from ice by melting and trapping with liquid he followed by analysis of n2 o2 ar 18oo2 and 15nn2 by continuous flow mass spectrometry the method was tested using glacier ice aufeis ice river icing and intrasedimental ground ice from sites in the canadian arctic o2 ar and n2 ar ratios clearly distinguish between atmospheric gas in glacial ice and gases from intrasedimental ground ice which are exsolved from freezing water 15nn2 and 18oo2 in glacier ice aufeis ice and intrasedimental ground ice do not show clear distinguishing trends as they are affected by various physical processes during formation such as gravitational settling excess air addition mixing with snow pack and respiration

aspects of climate variability in the north atlantic sector discussion and relation to the greenland ice sheet project 2 high resolution isotopic signal this paper reviews aspects of climate variability in the north atlantic sector that may influence the seasonal to decadal scale isotopic signal in the greenland ice sheet project 2 gisp2 ice core interpretation of the isotopic signal and its spatial applicability at the seasonal level requires investigation into synoptic scale climatology we discuss possible climatic influences of 1 likely source regions of precipitation reaching the gisp2 site 2 the characteristics of cyclone activity over the north atlantic sector and 3 changes in major atmospheric features such as the mean sea level icelandic low and azores high pressure systems the north atlantic oscillation and the baffin trough next we evaluate correlations between the gisp2 deuterium isotopic signal and coastal temperatures with atmospheric pressure patterns thereby deriving climatic interpretations of the high resolution isotopic record the gisp2 site is influenced by both the icelandic low to the southeast and davis strait baffin bay storms to the southwest and west the north atlantic oscillation influences the gisp2 isotopic signal through the seesaw in winter temperatures between west greenland and northern europe agreement in excursion directions of gisp2 isotopes and east greenland and iceland temperature records is associated with different positions of the baffin trough in winter and also with anomalous 500 mbar net geostrophic flow linkages between isotopic excursion direction and atmospheric variability need to be explored further by comparison with additional seasonal data sets from other greenland ice cores

granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

the use of porphyroblasts to resolve the history of macro scale structures an example from the robertson river metamorphics north eastern australia the successions of inclusion trail asymmetries defining foliation intersection inflection axes fias preserved in porphyroblasts document the geometry of deformation associated with folding and fabric development during discrete episodes of bulk shortening and suggest that deformation histories are commonly more complex than previously considered a succession of four fias trending ene wsw e w n s and ne sw has been distinguished based upon relative timing plus inclusion texture and orientation in the proterozoic robertson river metamorphics georgetown inlier qld australia the successions of asymmetries formed around these fias bear no relationship to the geometry of macroscale folds present in the area suggesting that these folds predate porphyroblast growth the widespread metamorphism and matrix fabric development the onset of regional macro scale folding may have begun soon after the deposition at around 1655 ma in the georgetown inlier as opposed to the previously suggested date of 1570 ma these folds were then amplified overturned and refolded during nnw sse n s e w and nw se directed regional bulk shortening earlier deformations were erased from the matrix because of bedding induced shearing reactivation on the limbs of pre existing macro scale folds four foliations s1 s4 identified in the matrix provided information about the youngest deformations preserved in these rocks these results also suggest that existing tectonothermal models regarding the ne australian craton should be reconsidered

timing of the acadian orogeny in northern new hampshire abstract new u pb geochronology constrains the timing of the acadian orogeny in the central maine terrane of northern new hampshire sixteen fractions of one to six grains each of zircon or monazite have been analyzed from six samples 1 an early syntectonic diorite that records the onset of the acadian 2 a schist a migmatite and two granites that together record the peak of the acadian and 3 a postkinematic pluton that records the end of the acadian zircon from the syntectonic wamsutta diorite gives a 207pb 206pb age of circa 408 ma the time at which the boundary between the deforming orogenic wedge and the foreland basin was in the vicinity of the presidential range this age agrees well with the emsian position of the northwest migrating acadian orogenic front and records the beginning of the acadian in this part of the central maine terrane we propose a possible acadian tectonic model that incorporates the geochronologic structural and stratigraphic data monazite from the schist migma

effect of wind scouring on climatic records from ice core oxygen isotope profiles removal of winter snow by wind explains why the annual mean oxygen isotope ratio at an ice divide is 2 5 less negative than it is 1 2 km downslope cores from two sites on an ice cap ellesmere island canada show past variations in the difference depending on where the ice originated relative to the scoured zone some ice core climatic records may need correction for scouring

prolonged acadian orogenesis revelations from foliation intersection axis fia controlled monazite dating of foliations in porphyroblasts and matrix numerous phases of garnet growth are revealed by detailed studies of foliation inflection intersection axes preserved in porphyroblasts fias in acadian metamorphic rocks in the chester dome region of vermont u s a a regionally consistent succession of four different fia tends in garnet porphyroblasts has been dated by analyzing monazite inclusions with an electron microprobe these monazite inclusions which lie within the various foliations that define the fias provide absolute ages for multiple periods of deformation and episodic phases of garnet growth the monazite inclusions reveal a progression in foliation ages from 431 2 to 349 3 million years within porphyroblasts and from 366 3 to 327 5 million years in pervasive matrix foliations three samples of schist reveal ages ranging through 75 to 70 million years from the cores of porphyroblasts through the medians to the rims and then into the matrix ages determined from monazite grains within garnet porphyroblasts link directly to periods of multiple deformation and episodic garnet growth defined by the fias the four fia sets began forming prior to 424 3 405 6 386 6 and 366 4 million years ago respectively thus multiple stages of garnet growth occurred throughout acadian deformation and metamorphism in vermont and orogenesis was far more prolonged than previously thought invariably garnet growth occurred early in the accompanying deformation event dating of monazite inclusions without careful separation of phases of garnet growth by fia studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural history analysis of monazite grains in the matrix alone will likely only present ages for the youngest events these ages can be amalgamated from grain to grain to yield apparently precise ages but such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis or the overall continuity of the deformation and metamorphism that accompanies plate motion fia and microstructural studies of inclusion trail successions allow numerous phases of garnet growth to be distinguished and then dated using monazite that has dissolved and regrown many times as multiple successive foliations developed this in turn allows very long and involved metamorphic histories to be fully integrated with similarly complex structural histories

granite ascent in convergent orogenic belts testing a model the common spatial relationship in convergent orogenic belts between a crustal scale shear zone system high grade metamorphic rocks and granites suggests a feedback relation between crustal anatexis and contractional deformation that helps granite extraction and focuses granite ascent such a feedback relation has been proposed for ascent of early devonian granites in west central maine this interpretation requires that deformation metamorphism and plutonism were synchronous we have determined precise u pb zircon and monazite ages that we interpret to record time of crystallization of syntectonic granite in metric to decametric sheets and kilometric plutons and of schlieric granite within migmatites ages are in the range ca 408 404 ma within 1 m y at 95 confidence limits these ages are similar to extant u pb monazite ages of ca 405 399 2 ma for syntectonic regional metamorphism in the same area the coincidence between the age of peak metamorphism and crystallization ages of granite shows tectonics metamorphism and magmatism were contemporaneous in support of the feedback model

the effect of microstructural and rheological heterogeneity on porphyroblast kinematics and bulk strength in porphyroblastic schists abstract the kinematic record and bulk viscous strength of polyphase rocks depend in part upon the relative strengths and distributions of rheologically distinct fabric elements here we explore the effects of microstructural and rheological heterogeneity in porphyroblastic schists electron backscatter diffraction and petrographic analyses reveal asymmetric microboudinage of staurolite indicating relative rotation of staurolite porphyroblasts synchronous with bulk non coaxial strain boudinage and relative rotation both require porphyroblast matrix shear coupling based on 2d optical observations the extent of the coupling appears related to the initial and boudinaged staurolite grain shape and orientation as well as the geometry of heterogeneities such as mica domains or shear bands we designed 2d finite element numerical models to assess the role of microstructural variation and rheological heterogeneity on the degree of porphyroblast matrix shear coupling and bulk viscous strength model results indicate that the bulk strength of a three phase system comprising inclusion weak domain and matrix is sensitive to the relative proximity of weak and strong domains particularly at high viscosity contrasts i e matrix weak 10 the threshold for bulk weakening below the matrix strength occurs over a narrow range of weak domain viscosities matrix weak 2 6 5 5 regardless of the relative abundance and spatial distribution of weak domains kinematic decoupling of porphyroblasts occurs at low viscosity contrasts when weak domains are proximal matrix weak 2 5 but for all other spatial distributions and modal abundances investigated kinematic decoupling occurs at viscosity contrasts of matrix weak 15 20 these data indicate that bulk weakening due to rheological heterogeneity is not necessarily coincident with kinematic decoupling

concentric slow cooling of a low p high t terrane evidence from 1600 1300 ma mica dates in the 1780 1700 ma black hills orogen south dakota u s a the crystalline core of the southern black hills south dakota exposes an extensive low p high t aureole of garnet to second sillimanite zone schists centered on the plutonic core of the 1715 ma harney peak granite hpg this paper demonstrates regional patterns of apparent ages observed for 52 40 ar 39 ar dates of muscovite and biotite in diverse rocks from across the 1000 km 2 metamorphic aureole and its plutonic center about 20 biotite dates sampled mostly near faults are influenced by excess 40 ar and obscure the regional trends the remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite with previously unrecognized elliptical age zones centered on the main hpg pluton and its outliers the regional pattern of 40 ar 39 ar cooling ages indicates non uniform slow cooling of the mid crust between 1600 1250 ma this scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite rb sr and apatite u pb which range from 1690 to 1550 ma and from 1700 to 1500 ma respectively to explain these results it is likely that ambient pre granite temperatures of the country rocks were 350 c at the 10 14 km depth of granite emplacement as previously proposed and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years alternatively or in addition the hpg and inner aureole were not uplifted until 1480 1330 ma whereupon they finally cooled through 300 350 c

a record of holocene summer climate from a canadian high arctic ice core melt layers in ice cores are formed by melting on the snow pack surface which produces water that percolates down and refreezes in the colder snow layers below melt layers are distinguished by their bubble texture because more ice forms in the snow pack in warm summers than in cool summers the changing concentration with depth of melt layers in snow pits1 2 and ice cores3 5 is an indicator of past summer climate here we report on studies of melt layers in two canadian high arctic surface to bedrock ice cores although changes in elevation of the drill site owing to isostatic depression and uplift may contribute up to 40 of the total climatic signal recorded in the melt layers we interpret their varying concentrations in terms of changing summer temperatures over the past 10 000 years the warmest summers occurred 8 10 kyr ago and the coldest only 150 years ago the summers over the past 100 years have been the warmest for more than 1 000 years but are still not as warm as those of the early holocene the melt layer record is in broad agreement with geological records of glacier retreat during the holocene6 8 indicating that these data also contain information of regional significance

isotope thermometry in melt affected ice cores 1 a statistically significant relationship is observed between stable water isotopes 18o and melt amounts in a melt affected firn core ssummit taken from the prince of wales icefield ellesmere island canada by contrast a low melt firn core taken from a higher elevation higher latitude location on the same icefield shows no relationship between these variables we interpret this as evidence for meltwater induced isotopic enrichment at ssummit a percent melt based correction slope is applied to isotopic values from ssummit uncorrected and corrected temperature records derived from the raw and corrected 18o values are compared to bias corrected temperature data from the ncep reanalysis improvements are observed in the isotopic reconstruction of ssummit annual precipitation weighted temperatures when we correct for meltwater enrichment with a reduction from 0 6 c to 0 0 c in the mean annual error and a decrease in root mean square error from 1 8 c to 1 6 c the correction factor appears to overcorrect isotopic modification during high melt years such as 1999 during which ssummit experienced nearly 70 more melt than the average from 1975 to 2000 excluding 1999 data from the correction analysis results in a slight reduction in mean absolute error from 1 4 c to 1 3 c these results suggest that melt induced isotopic modification cannot be corrected in very high melt years

natural and anthropogenic levels of tritium in a canadian arctic ice core agassiz ice cap ellesmere island and comparison with other radionuclides

evidence for decreasing south asian summer monsoon in the past 160 years from varved sediment in lake xinluhai tibetan plateau 1 we report glacial varves in the sediment of lake xinluhai tibetan plateau independent data of 137cs and 210pb indicate that these are annually deposited varves varves appear as rhythmic units of light colored silt layer capped by a dark clay layer under microscope varve thickness in lake xinluhai is sensitive to precipitation because sediment accumulation is strongly affected by monsoon rainfall in the area a general decreasing trend can be observed in the varve thickness over the past 160 years spectral analyses of the varve record are dominated by cycles which are similar to enso periodicities it implies that the decreasing trend of the south asia monsoon may be linking with enso spatially the decreasing trend can be observed across different proxy records in the south of the tibetan plateau although arguments still remain for the dynamic mechanisms and spatial rainfall difference the south asian summer monsoon could be weakened due to rising temperatures

spatial covariance of water isotope records in a global network of ice cores spanning twentieth century climate change 1 estimating the spatial extent of past climate changes has been an ongoing challenge for paleoclimatology for such estimates to be made with confidence it is important to establish an understanding of the spatial coherence of proxy records during an interval of known climate change we use water stable isotopes from high resolution ice cores and twentieth century observations of sea level pressures and sea surface temperatures to assess the covariance among isotopic records and its link to organized patterns of climate variability covarying signals in the cores are identified using empirical orthogonal function analysis results from regression analysis show that the leading signals are consistent with key climate patterns including the northern atlantic oscillation and southern annular mode and variability in tropical pacific sea surface temperatures associated with the el nino southern oscillation patterns that have recently been identified in instrumental data such as positive tropical pacific sst anomalies associated with the negative phase of the sam are evident in the ice cores these explanations for the variance of stable isotopes are consistent with recent studies using isotope enabled general circulation models and provide a physical basis for interpreting the observed isotopic signals while there is also a global change signal that is evident when analyzing the records collectively there are some limitations in reconstructing global temperatures due to the geographic coverage of the available records and the current lack of modeling studies to explain the observed global scale changes still water stable isotope ratios preserved in ice cores provide a sufficiently rich sampling of large scale climate variability that they can be more widely used in physically based paleoclimate reconstructions covering the last millennium and other periods

the mechanism of ascent and emplacement of granite magma during transpression a syntectonic granite paradigm we propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern appalachians in the study area in western maine strain was distributed heterogeneously during devonian acadian transpression metasedimentary rocks migmatites at high grades record two contrasting types of finite strain in zones that alternate across strike rocks in both types of zones have a penetrative moderately to steeply ne plunging mineral elongation lineation defined by bladed muscovite fibrolite sillimanite at high grades in straight belts of enhanced deformation rocks have s l fabrics that record apparent flattening to plane strain apparent flattening zones afzs but rocks between these belts have l s fabrics that record apparent constriction apparent constriction zones aczs at metamorphic grades above the contemporary solidus rocks in afzs developed stromatic structure in migmatite which suggests that percolative flow of melt occurred along the evolving flattening fabric stromatic migmatites are intruded by concordant to weakly discordant m scale composite sheet like bodies of granite to suggest magma transport in planar conduits through the afz rocks inhomogeneous migmatite is found in the intervening aczs which suggests migration of partially molten material through these zones en masse probably by melt assisted granular flow inhomogeneous migmatites are intruded by irregular m scale bodies of granite that vary from elongate to sub circular in plan view and seem cylindrical in three dimensions these bodies apparently plunge to the northeast parallel to the regional mineral elongation lineation to suggest magma transport in pipe like conduits through the acz rocks we postulate that the form of magma ascent conduits was deformation controlled and was governed by the contemporaneous strain partitioning magma ascent in planar and pipe like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform within this hot corridor it is the difference in temperature between melt producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust at the wet solidus we suggest melt flows obliquely toward axial culminations in the thermal antiform which are sites of melt accumulation and perturbations from which magma may escape to form plutons batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust or ascent may become inhibited with decreasing depth by thermal arrest and solidification if the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction melt pressure ultimately may lead to sub horizontal magma fracture or viscous flow of wall rocks may allow lateral spreading the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduit as the thermal antiform decays the height of sub solidus crust that separates the deepest part of the pluton from the anatectic zone increases consequently pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry

episodic metamorphism and deformation in low pressure high temperature terranes in many cases low pressure high temperature metamorphism must be the consequence of advective heat transfer e g the ascent of granitic magma whereas such heating mechanisms are necessarily short lived the lifetime of the underlying orogenic processes is likely to be much longer and it is conceivable that during the evolution of an orogen repeated transient heating at shallow crustal levels is caused by episodes of segregation and upward passage of melts generated at deep crustal levels the number and timing of these events would depend on such factors as critical melt segregation volume moho temperature and strain rate evolution we investigate some of the controls on this multiple event superposition using a simple thermomechanical model designed to predict simultaneously the strain rate and the isostatic and thermal evolution of convergent orogenesis subject to a constant driving force an appealing aspect of this formulation is that in contrast to models that have attempted to explain event cyclicity through episodic processes acting on the orogenic boundaries it accounts for the repeated occurrence of events through processes inherent to the orogenic system itself

deformation partitioning during transpression in response to early devonian oblique convergence northern appalachian orogen usa abstract transpressive deformation was distributed heterogeneously within the central maine belt shear zone system which formed in response to early devonian oblique convergence during the acadian orogeny in the northern appalachians straight belts are characterized by tight folds s l fabrics and sub parallel form lines and asymmetric structures that together indicate dextral se side up kinematics in contrast intervening zones between straight belts are characterized by open folds and l s fabrics within both types of zone metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade including a penetrative moderately to steeply ne plunging mineral lineation thus we interpret accumulation of plastic deformation and regional metamorphic re crystallization to have been synchronous across the central maine belt shear zone system discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in straight belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the central maine belt shear zone system perturbations in the flow were caused by serially developed thrust ramp anticlines in the stratigraphic succession immediately above the avalon like basement at which decollement of the shear zone system was initially rooted general shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow in contrast deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow part of the thickening stratigraphic succession exceeded tsolidus reflected by the occurrence of migmatites and granites the latter were partly sourced from the underlying avalon like basement that was involved in the deformation and melting

distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral staircase and sigmoidal inclusion trails in garnet schists from the appalachian orogen in south east vermont have undergone multiple phases of garnet growth these phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes fias of foliations preserved as inclusion trails in garnet porphyroblasts the relative timing of different generations of fias is determined from samples containing porphyroblasts with two or three differently trending fias developed outwards from core to rim multi fia porphyroblasts schists from south east vermont show a consistent pattern of relative clockwise rotation of fia trends from oldest to youngest four populations or sets of fias can be distinguished on the basis of their relative timings and trends from oldest to youngest the four sets have modal peaks trending sw ne w e nnw sse and ssw nne these peaks show that each of the four fia sets has a statistically consistent trend at all scales across a 35 125 km area containing numerous mesoscopic and macroscopic folds the fias of set 4 are defined by inclusion trails that are continuous with matrix foliations have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures conversely sets 1 to 3 are oblique to and pre date most matrix foliations and folds all four fia sets occur in siluro devonian rocks and must have formed in the acadian orogeny the lack of statistically significant differences in the distribution of fia trends across the study area and their consistent relative timings in multi fia porphyroblasts despite a complex regional deformation history involving numerous phases of folding at all scales suggest the porphyroblasts have not rotated relative to one another the change in fia trend with time resulted from rotation of the kinematic reference frame of bulk flow possibly as a consequence of the reorganization of lithospheric plates responsible for acadian orogenesis recognition of distinct generations of fias provides a means of distinguishing different phases of porphyroblast growth four periods of garnet porphyroblast growth occurred in the schists of south east vermont this growth was heterogeneously distributed on the cm2 m2 scale no single porphyroblast records all stages of growth and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth factors that may have been responsible for switching porphyroblast growth on and off at this scale include i subtle differences in bulk chemical composition ii oscillating levels of heat owing to the buffering effect of endothermic garnet forming reactions iii channelized infiltration of fluids with localized fluid buffering of bulk composition and iv cyclic controls on the rates of diffusion and material transport of reactants either by channelized fluid flow or by a changing pattern of microfracturing during foliation development consistency in fia trend and relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events or even phases of metamorphism within a progressive tectonothermal event along and across orogens using a consistent pattern of core to rim changes in fia trend multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished allowing correlation of different phases of growth around and across macroscopic folds the relative timing of growth of different porphyroblastic minerals can also be quantitatively determined using fia data and correlated around and across macroscopic folds conceptually the paragenetic history preserved in each generation of porphyroblast growth in the form of chemical zoning and the minerals in inclusion trails could be combined to produce a more detailed p t t deformation path than previously determined

u pb single zircon pb evaporation and sm nd isotopic study of a granulite domain in se madagascar madagascar is a poorly known precambrian domain at the eastern border of the mozambique belt strongly affected by the pan african orogeny this large island represents a key part of gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent in the southeastern zone of the island metamorphism has reached high temperature and low pressure conditions 850 c and 5 kbar with moderate retrogression a u pb conventional multigrain single zircon stepwise pb evaporation and sm nd geochronological study on meta felsic rocks has shown that basement of at least lower proterozoic age was strongly metamorphosed around 570 580 ma this tectometamorphic event nearly obliterated any pre pan african zircon isotopic memory the granulite facies metamorphism is associated with the syn metamorphic emplacement of the anosyan charnock ites and granites a strong channelized fluid contribution at 545 ma has produced the re crystallization of biotite apatite

stratigraphic analysis of an ice core from the prince of wales icefield ellesmere island arctic canada using digital image analysis high resolution density past summer warmth reconstruction and melt effect on ice core solid conductivity 1 high resolution 1 mm stratigraphic information was derived from digital image analysis of an ice core from the prince of wales pow icefield central ellesmere island canada following careful image processing a profile of ice core transmitted light was derived from the greyscale images and used to reconstruct high resolution density variations for the unfractured sections of the core images were further classified into infiltration and glacier ice using an automatic thresholding procedure and were converted to a high resolution melt percentage index the mean annual melt percentage over the last 580 years was 9 and melting occurred in 8 years out of 10 melting obliterated most of the original depositional sequence and seasonal density cycles were mostly unrecognizable the ice core solid conductivity was greater and more variable in melt features than in glacier ice owing to washout of strong acids by meltwater elution and chemical enrichment upon refreezing this hindered the identification of acid volcanic layers and further compromised dating by annual layer counting comparison of the melt record with those from other arctic ice caps shows that the melt temperature relationship on pow icefield is site specific we speculate that this is due to the peculiar position of the icefield which rests on the periphery of the baffin bay maritime climate zone and to the proximity of the north open water polynya which controls snow accumulation variability on the icefield and affects the melt percentage index

mid to late holocene hydroclimatic and geochemical records from the varved sediments of east lake cape bounty canadian high arctic abstract a long sedimentary sequence from east lake cape bounty melville island 74 55 n 109 30 w contains a 4200 year long clastic varved record of paleohydrologic variations at high resolution sedimentary elemental geochemistry from micro x ray fluorescence xrf and sediment fabric variability reflect changes in sediment sources and lacustrine conditions through time the sedimentary environment progressed from marine in the mid holocene to estuarian from 2195 bc to 243 ad to fully lacustrine source after 244 ad correlation with local meteorological data indicates that varve thickness vt is positively correlated with snow depth on may 1st and negatively correlated with mean sept may temperatures our paleoclimatic reconstruction from vt series revealed high snow accumulation and warm sept may months before 1350 bc and a period of low snow accumulation and cold sept may between 1600 1900 ad that may correspond to the little ice age the general trends of vt series from cape bounty are in phase with the 18o series in agassiz ice cap and in anti phase with the vt series from lower murray lake in the northeastern of queen s elizabeth islands qei low mean arctic temperatures coincide with clusters of high sediment yield events at east nicolay and south sawtooth lakes especially during 1600 1750 ad and 1810 1910 ad the east lake record also exhibits the signature of the atlantic multidecadal oscillation amo for periods 600 850 ad 1400 1550 ad and 1750 1850 ad

melt features in ice cores from site j southern greenland some implications for summer climate since ad 1550 horizontal and vertical distributions of melt features ice layers were examined using two ice cores 206 6 and 101 5 m deep 1 m apart from site j 66 51 9 n 46 15 9 w 2030 m a s l the temperature at 10 m was 16 3 c we observed 2804 melt features with a total thickness of 30 32 m in the 206 6 m core corresponding to 16 4 by volume of the ice equivalent core length horizontal distribution of melt features was examined by correlating melt feature thicknesses in the two cores the correlation coefficient was 0 71 n 514 for each melt feature in the two cores it was maximum for data passed through 5 and 40 year low pass filters a significant relationship p 0 005 n 36 was obtained for the vertical distribution of melt features and the june temperature on the west coast of greenland jakobshavn using this june temperatures at jakobshavn since 1550 were estimated there are three periods 1685 1705 1835 70 and 1933 45 during which mean june temperatures clearly decreased when they were estimated to he 0 1 0 4 and 0 2 c lower than the average for the whole period 1550 1989 the first two cold periods have been identified in melt features of the dye 3 and devon island ice cores and in a tree ring profile from yukon territory canada melt feature percentages in the site j ice core have increased since about 1945 probably reflecting summer temperature warming on the ice sheet

high resolution multiproxy climatic records from ice cores tree rings corals and documentary sources using eigenvector techniques and maps assessment of recovered signal and errors palaeotemperature sensitive series from tree rings ice cores corals and documentary sources are combined to producepalaeo summer temperaturereconstructions ad 1970 1761 and geographical eigenvector eof maps in both hemispheres they are compared favourably to those of existing summer temperature average series there are 51 palaeoseries in the northern hemisphere mostly north of 40 n and 16 series in the southern hemisphere the statistics and significance of the palaeoreconstructions are examined by 1 finding the correlation coefficient palaeo to measured as a function of the number and geographical distribution of the palaeo series 2 developing and running a multiproxy model that generates pseudo series containing a signal and the same types and amounts of noise found in the various real palaeo series the model reproduces the measured correlation coefficients and the eigenvector s eof explained variances as functions of the number of sites about 77 of the signal variance can be reco

some comments on climatic reconstructions from ice cores drilled in areas of high melt poor consideration has been given in many arctic circum polar ice core studies to the effect of summer snow melt on chemistry stable isotope concentrations and time scales many of these corps are drilled close to the firn line where melt is intense some come from below the firn line where accumulation is solely in the form of super imposed ice in all cases seasonal signals are reduced or removed and in some time gaps develop during periods of excessive melting which situate the drill site in the ablation zone consequently cross correlations of assumed synchronous events among the cores are invalid so that time scales along the same cores differ between authors by factors of over 2 many so called climatic signals are imaginary rather than real by reference to published analyses of cores from the superimposed ice zone on devon ice cap koerner 1970 and meighen ice cap koerner and paterson 1974 it is shown how melt affects all the normally well established ice core proxies and leads to their misinterpretation despite these limitations the cores can give valuable low resolution records for all or part of the holocene they show that the thermal maximum in the circum polar arctic occurred in the early holocene this maximum effected negative balances on all the ice caps and removed the smaller ones cooler conditions in the second half of the holocene have caused the regrowth of these same ice caps

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

volcanic forcing of climate over the past 1500 years an improved ice core based index for climate models 1 understanding natural causes of climate change is vital to evaluate the relative impacts of human pollution and land surface modification on climate we have investigated one of the most important natural causes of climate change volcanic eruptions by using 54 ice core records from both the arctic and antarctica our recently collected suite of ice core data more than double the number of cores ever used before reduces errors inherent in reconstructions based on a single or small number of cores which enables us to obtain much higher accuracy in both detection of events and quantification of the radiative effects we extracted volcanic deposition signals from each ice core record by applying a high pass loess filter to the time series and examining peaks that exceed twice the 31 year running median absolute deviation we then studied the spatial pattern of volcanic sulfate deposition on greenland and antarctica and combined this knowledge with a new understanding of stratospheric transport of volcanic aerosols to produce a forcing data set as a function of month latitude and altitude for the past 1500 years we estimated the uncertainties associated with the choice of volcanic signal extraction criteria ice core sulfate deposition to stratospheric loading calibration factor and the season for the eruptions without a recorded month we forced an energy balance climate model with this new volcanic forcing data set together with solar and anthropogenic forcing to simulate the large scale temperature response the results agree well with instrumental observations for the past 150 years and with proxy records for the entire period through better characterization of the natural causes of climate change this new data set will lead to improved prediction of anthropogenic impacts on climate the new data set of stratospheric sulfate injections from volcanic eruptions for the past 1500 years as a function of latitude altitude and month is available for download in a format suitable for forcing general circulation models of the climate system

format and philosophy for collecting compiling and reporting microprobe monazite ages microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains down to 5 m or less in structural and petrologic context this paper presents an analytical strategy presentation format and error considerations for microprobe monazite dating the strategy involves high resolution compositional mapping to delineate compositional domains within monazite crystals then for each compositional domain a series of th u and pb analyses are made and a single date and error are calculated the number of analyses in each domain is determined by the desired statistical precision of the date results from several monazite grains are typically combined and along with textural relationships are used to build an argument that the dates constrain the age of a deformation or metamorphic event the total error involves three components short term random error dominated by counting statistical uncertainty short term systematic error uncertainty in background correction conductive coating variation and calibration and long term systematic error uncertainty in standard composition mass absorption factors decay constants etc in homogeneous compositional domains short term random errors 2 of less than 10 m y can be obtained from five to ten analyses however short term systematic error mainly background estimation uncertainty would typically result in a doubling of the magnitude of random error microprobe dates are presented as a single gaussian probability distribution for each domain along with representative compositional maps it is recommended that a consistency standard be analyzed during each analytical session and the results be reported along with those from the unknown this proposed strategy and format are compatible with those of other geochronological techniques they incorporate analytical limitations associated with trace as opposed to major element microprobe analysis and will allow better comparisons to be made between labs and between different geochronological techniques

the effects of wind on δ 18o and accumulation give an inferred record of seasonal δ amplitude from the agassiz ice cap ellesmere island canada

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

diatom community response to multiple scales of holocene climate variability in a small lake on victoria island nwt canada abstract a lake sediment core spanning 9900 years collected from a small lake on western victoria island nwt canada provides a high resolution record of diatom community dynamics over the holocene ten radiocarbon dates and 210 pb dating provided the core chronology loss on ignition loi magnetic susceptibility biogenic silica content and diatom concentrations provided information on changes in the sedimentary environment loi gradually increased over the holocene whereas magnetic susceptibility showed an inverse trend biogenic silica content showed three distinct peaks spaced approximately 3000 years apart major shifts in diatom assemblages occurred at 8100 8000 5800 5700 and 3800 3500 cal yr bp there is evidence of diatom community response to centennial scale variations such as the medieval warm period 1000 700 cal yr bp little ice age 800 150 cal yr bp and recent warming although recent changes in diatom community composition productivity and species richness are apparent they were surpassed at other periods throughout the holocene variations of the taxa within the genera staurosira pseudostaurosira fragilaria and staurosirella usually combined into one genus in arctic lake sediment studies suggest these taxa provide useful insight into paleonvironmental questions

a record of climate over the last millennium based on varved lake sediments from the canadian high arctic a varved sediment record that extends back over the last millennium was recovered from lower murray lake northern ellesmere island nunavut canada 81 20 n 69 30 w flatbed scan images and back scattered electron images were analysed to provide varve thickness and other quantitative sedimentary indices on an annual basis in many studies of lakes from the high arctic varve thickness is a good proxy for summer temperature and we interpret the lower murray lake varves in this way on that basis the lower murray lake varve thickness record suggests that summer temperatures in recent decades were among the warmest of the last millennium comparable with conditions that last occurred in the early twelfth and late thirteenth centuries but estimates based on the sediment accumulation rate do not show such a recent increase the coldest conditions of the little ice age were experienced from ad 1700 to the mid nineteenth century when extensive ice cover on the lake led to widespread anoxic conditions in the deepest parts of the lake basin an overall decline in median grain size over the last 1000 years indicates a reduction in the energy available to transport sediment to the lake many of these features of the record are also observed in other palaeoclimatic records from the north american arctic the very recent appearance of the diatom campylodiscus which was not observed through out the record of the last millennium suggests that a new threshold in the ontogenetic development of the lake has now been passed

signal and noise in four ice core records from the agassiz ice cap ellesmere island canada details of the last millennium for stable isotopes melt and solid conductivity four ice cores and two deep pit auger sequences from the top of the agassiz ice cap have been variously analysed for o18 ecm solid conductivity and ice melt layer stratigraphy the high resolution data are presented on time scales covering about the last 1000 years the 8 time series are compared and the noise examined in terms of snow drifting and wind scouring of winter snow from the higher exposed parts of flow lines the highly scoured 8 records reflect a mixture of summer biased conditions and the amount of winter scoured snow sites away from the ridges and domes are not scoured the signal part of the scoured and unscoured sites is obtained by stacking averaging normalized series the stacked unscoured series of 8 and accumulation are compared to similar stacked series from central greenland for the last century of annual averages the comparison is surprisingly good suggesting that at these resolutions low noise 8 series have some coherence over fairly large distances the percentage melt

high radiogenic heat producing granites and metamorphism an example from the western mount isa inlier australia the origins of metamorphism 600 c and 3 4 kbar in the western mount isa inlier australia remain controversial for a number of reasons 1 no synmetamorphic intrusive bodies can be recognized 2 high temperatures appear to be sustained for periods 100 m y and 3 metamorphism follows an extended phase of thermal subsidence we show that the burial of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the mount isa group deposited in response to repeated rift sag cycles was capable of generating steep upper crustal thermal gradients immediately prior to the isan orogeny these gradients are appropriate to peak metamorphic conditions such that the ensuing isan orogeny required no significant additional heat input this result is significant in that it may provide a mechanism for understanding the origins of high temperature metamorphism in other terranes where the involvement of transient heating is not obvious

regional scale foliation reactivation and re use during formation of a macroscopic fold in the robertson river metamorphics north queensland australia abstract the rocks defining a macroscale antiform 25 km2 in the proterozoic robertson river metamorphics have been affected by four deformations of distinctive style with the fourth deformation d4 being responsible for formation of the macroscale geometry of the fold redistribution of progressive shearing strain due to strain partitioning during progressive d4 deformation resulted in the accommodation of d4 shearing strain along s2 differentiated crenulation cleavages that had been synchronously rotated into favourable orientations on the antiformal limbs this commonly resulted in reactivation of s2 because shearing during d4 was in a sense that was antithetic relative to that on the bulk scale of the fold continued d4 deformation caused unfolding of d2 crenulations resulting in straightening of sigmoidally folded s1 and its rotation toward the axial plane of the synchronously forming macroscopic d4 fold in zones where the sense of shear during d4 was the same as that operating on the bulk scale of the fold i e synthetic d4 shearing strain was accommodated dominantly by the favourably oriented approximately axial planar s1 fissility in zones where the progressive shearing component of d4 deformation was relatively more intense the s2 cleavage was also rotated into parallelism with s4 and was also re used as opposed to reactivated detailed microstructural analysis particularly of porphyroblast matrix relationships combined with field observations have resolved the processes operating during folding and reveal that despite the intensity of d4 deformation a separate cross cutting s4 cleavage has rarely been produced at either the meso or microscale similar processes have probably operated at all scales in other orogenic belts

reevaluation of the reconstruction of summer temperatures from melt features in belukha ice cores siberian altai 1 in a previous study past summer temperatures were reconstructed from melt features in the belukha ice core siberian altai we evaluated the climatic representativeness of net accumulation and melt features by comparing two belukha ice cores retrieved at neighboring sites by different institutions and dated by different methods melt features in both cores showed a significant correlation but the trends of net accumulation were different between the cores melt features corresponded to the retreat rate of a glacier terminus in a neighboring mountain range these findings demonstrate the spatial representativeness of melt features in the ice cores we reevaluated an equation formulated for reconstructions of summer temperature as used in a previous study and found that it underestimates temperature we propose an alternative equation to obtain more reliable summer temperatures from melt features and net accumulation records for the period from 1914 to 2003

a long lived metamorphic history in the contact aureole of the mooselookmeguntic pluton revealed by in situ dating of monazite grains preserved as inclusions in staurolite porphyroblasts emplacement of the mooselookmeguntic pluton located in the western maine region of the northern appalachians was thought to have occurred towards the end of the acadian deformation at around 370 ma crystallization ages from different parts of the pluton suggest a more sequential emplacement history over a period of c 20 myr foliation inflection intersection axes fias within porphyroblasts from its aureole reveal at least five periods of garnet and staurolite growth the orientation of fias in both garnet and staurolite porphyroblasts trend successively from ese wnw nnw sse e w ene wsw to ne sw electron probe microanalysis dating of monazite grains included in staurolite porphyroblasts containing one of these five periods of fia development reveals a succession of apparent ages from 410 ma to 345 ma a similar spread of crystallization ages can be observed for plutons from maine and adjacent regions this succession indicates that deformation and metamorphism began well before and continued long after what is classically regarded as the acadian orogeny the thermal structure of the orogen progressively evolved to enable pluton emplacement and it continued to develop afterwards with magmatic fluids still forming at depth

nature of the crust in maine usa evidence from the sebago batholith neodymium and lead isotope and elemental data are presented for the sebago batholith 293 2 ma the largest exposed granite in new england the batholith is lithologically homogeneous yet internally heterogeneous with respect to rare earth elements ree and nd isotopic composition two mica granites in the southern central portion of the batholith group 1 are characterized by ree patterns with uniform shapes cen ybn chondrite normalized 9 4 19 and eu eu eu anomaly 0 27 0 42 and nd t 3 1 to 2 1 peripheral two mica granites group 2 spatially associated with stromatic and schlieric migmatites have a wider range of total ree contents and patterns with variable shapes cen ybn 6 1 67 eu eu 0 20 0 46 and nd t 5 6 to 2 8 the heterogeneous ree character of the group 2 granites records the effects of magmatic differentiation that involved monazite coarse grained leucogranites and aplites have kinked ree patterns and low total ree but have nd isotope systematics similar to group 2 granites with nd t 5 5 to 4 7 rare biotite granites have steep ree patterns cen ybn 51 61 eu eu 0 32 0 84 and nd t 4 6 to 3 8 the two mica granites have a restricted range in initial pb isotopic composition 206pb 204pb 18 41 18 75 207pb 204pb 15 60 15 68 208pb 204pb 38 21 38 55 requiring and old high u pb but not th u source component the nd isotope data are consistent with magma derivation from two sources avalon like crust nd 3 and central maine belt metasedimentary rocks nd 4 without material input from the mantle the variations in isotope systematics and ree patterns are inconsistent with models of disequilibrium melting which involved monazite

a deformation partitioning approach to resolving the sequence of fold events and the orientations in which they formed across multiply deformed large scale regions abstract regional distributions of axial plane trends retain information on the orientation in which successive generations formed because multi scale partitioning results in most orogenic belts preserving subsequently undeformed portions of all large scale folds at depths greater than 10 km within orogens successions of regional folds are accompanied by the sequential development of crenulation hinges in pelites which are commonly overgrown early during their development by successive generations of porphyroblasts consequently the original trends of the axial planes of these folds are preserved within the distribution of foliation inflection intersection axes within porphyroblasts fias peaks in the distribution of fia trends in western maine predominantly coincide with peaks in the distribution of trends of the axial planes of macroscopic and regional folds the wnw ese 420 ma n s 408 10 ma w e 388 9 ma wsw ene 372 5 ma sw ne 353 4 ma succession of fia peaks defines the sequence of folds and accords with map scale overprinting relationships this quantitative approach to interpreting fold successions in multiply deformed terrains resolves timing where overprinting criteria are rare uncertain or obliterated by younger events in portions of the orogen significantly lengthy detailed histories of structural development can be extracted from a small area containing porphyroblastic rocks and applied to very large scale regions

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

structure and origin of holocene cold events abstract the present interglacial the holocene spans the period of the last 11 700 years it has sustained the growth and development of modern society the millennial scale decreasing solar insolation in the northern hemisphere summer lead to northern hemisphere cooling a southern shift of the intertropical convergence zone itcz and a weakening of the northern hemisphere summer monsoon systems on the multidecadal to multicentury scale periods of more stable and warmer climate were interrupted by several cold relapses at least in the northern hemisphere extra tropical area based on carefully selected 10 000 year long time series of temperature and humidity precipitation as well as reconstructions of glacier advances the spatiotemporal pattern of six cold relapses during the last 10 000 years was analysed and presented in form of a holocene climate atlas hoclat see http www oeschger unibe ch research projects holocene atlas a clear cyclicity was not found and the spatiotemporal variability of temperature and humidity precipitation during the six specific cold events 8200 6300 4700 2700 1550 and 550 years bp was very high different dynamical processes such as meltwater flux into the north atlantic low solar activity explosive volcanic eruptions and fluctuations of the thermohaline circulation likely played a major role in addition internal dynamics in the north atlantic and pacific area including their complex interaction were likely involved

an assessment of the regional distribution of the oxygen isotope ratio in northeastern canada a compilation of mean values of the oxygen isotope ratio relative to standard mean ocean water for 22 sites representative of conditions in north eastern canada is complemented with data on mean annual surface temperature latitude surface elevation and mean annual shortest distance to open ocean denoted by the 10 sea ice concentration boundary stepwise regression analysis is used to develop a multivariate model suitable to infer the distribution of 6 1 0 in an area of complex topography and possibly mixed source of advected water vapor the best model is produced by a run in the backward mode at the 95 confidence level in which only temperature latitude and distance to the open ocean remain in the model the correlation coefficient is 0 915 the adjusted coefficient of determination is 0 809 the root mean square residual is 1 62 this model is similar to the best 6180 predictive model derived elsewhere for greenland suggesting a common principal source of advected moisture

ice cores as an index of global volcanism from 1850 to the present to evaluate an important cause of past climate change climate researchers need reliable estimates of volcanic aerosol loading in the atmosphere previous indices the dust veil index volcanic explosivity index and those by mitchell 1970 sato et al 1993 and khmelevtsov et al unpublished manuscript 1993 all have drawbacks ice core acidity and sulfate records because they contain physical evidence of atmospheric loading are a promising source of information on past volcanic aerosols but these records contain large nonvolcanic signals as well we have compared northern hemisphere nh and southern hemisphere sh annual average versions of five indices with eight nh and six sh ice core records for the period 1850 to the present in an attempt to identify the volcanic signal common to all records the indices are all highly correlated with each other for the nh although the individual ice core records are in general not well correlated with each other or with any of the indices a composite derived from averaging the cores the ice core volcano index ivi shows promise as a new index of volcanic aerosol loading this new index correlates well with the existing non ice core volcanic indices and with high frequency temperature records for the sh the individual ice cores and indices are better correlated the shivi is again highly correlated with all indices and individual ice cores but not with high frequency temperature records for both hemispheres the southern oscillation index shows no significant correlation with the volcanic indices ice cores or the ivi thus providing no evidence for the impact of volcanic eruptions on el nino southern oscillation events

precision estimation in electron microprobe monazite dating repeated measurements versus statistical poisson based calculations quoted errors on electron microprobe monazite ages may not reflect true errors arising from natural variation in the data and potentially lead to incorrect or misleading geological interpretations the precision of single spot analyses where ages are calculated from the pb u and th concentrations from a single analysis and then combined with others to generate an average age are most significant in this regard a comparison between the classical method based on a description of the variability of multiple measurements and a more electron microprobe specific method using poisson counting statistics shows that the results from each will generally be different sixty analyses from a compositionally homogenous monazite grain were used as a data set to enable the two methods to be compared and the results show that poisson statistics underestimates the real precision of electron microprobe analyses more accurate estimates of the true precision are achievable using confidence intervals derived from standard errors of the mean as long as homogenous grains and or domains are analysed

fia trends along the precambrian rocky mountains a new approach to timing continental docking a similar succession of foliation inflection intersection axis fias trends preserved within porphyroblasts is present in two areas separated by 200 km along the rocky mountains the precambrian rocks in central colorado and northern new mexico were affected by deformation and metamorphism from 1506 to 1366 ma a succession of five fias trending w e ssw nne nnw sse nw se and wsw ene is distinguished in central colorado and dated at 1506 15 ma 1467 23 ma 1425 18 ma not dated and 1366 20 ma respectively to the south in northern new mexico a succession of five fias trending ssw nne wnw ese nnw sse nw se and wsw ene is distinguished and dated at 1482 48 ma 1448 12 ma 1422 35 ma not dated and 1394 22 ma the excellent correlation of the sequence of fia trends and their ages between regions reveals a sixfold fia succession across the region with the first developed fia set in central colorado not present in northern new mexico and the third fia set in the region not present in central colorado preferential partitioning of w e trending deformation into the central colorado region 1506 15 ma was followed by ssw nne trending deformation that affected both regions at 1470 20 ma however preferential partitioning of wnw ese trending deformation into northern new mexico at 1448 12 ma left central colorado unaffected both regions were then affected by the three remaining periods of orogenesis the first trending nnw sse at 1424 15 ma followed by one trending nw se that has not yet been dated and then one trending wsw ene at 1390 19 ma this suggests that the yavapai terrane was tectonized at 1506 ma prior to amalgamation with the mazatzal terrane 1470 ma subsequent orogenesis was initially partitioned preferentially into the mazatzal terrane but the following three periods of tectonism affected both terranes in a similar manner

radical changes in bulk shortening directions during orogenesis significance for progressive development of regional folds and thrusts abstract polyphase folding and metamorphism of rocks in the eastern mount isa inlier resulted from four distinct orogenic cycles these consist of ne sw shortening o00 from 1672 to 1648 ma involving a succession of five phases of deformation which was responsible for the development of the first generation nw se striking macroscopic folds plus cleavages that are mainly only preserved in porphyroblasts n s shortening o1 followed to 1600 ma involving at least three phases of deformation this period of n s directed orogenesis rotated large portions of the nw se trending regional folds into w e trends and the dominance of locally well preserved w e trending sub vertical matrix foliations as well as the intensification of foliation parallel to bedding subsequent w e shortening o2 from 1600 to 1515 ma was accompanied by the development of n s trending folds these dominate the mount isa inlier and where best developed involved the formation of at least three near orthogonal foliations where these large scale folds regionally are weakly developed or not visible only one sub vertical foliation has developed finally nw se shortening o3 produced locally developed sw ne trending folds and weak crenulations until 1500 ma the asymmetry of steep to gentle pitch changes in inclusion trails preserved in garnet porphyroblasts that developed during o00 indicates thrusting to the sw similar inclusion trail asymmetries in garnet staurolite and andalusite porphyroblasts that developed in o1 indicate thrusting to the n confirming a change in thrust direction across the wonga duchess belt to the w formation of the meerenurker roof thrust near mount isa by thrusting to the sw during o00 with subsequent development of the kokkalukkanurker duplex below by thrusting to the s during o1 reduces the displacement required for the roof thrust from 234 to 65 kms foliations preserved in o00 garnets are truncated by foliations associated with the development of o1 in the matrix o1 garnet staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during o2 the inclusion trails entrapped within o00 and o1 generated porphyroblasts reveal that the foliations that progressively developed in the matrix during these orogenies were composite fabrics resulting from the effects of multiple discrete orthogonal increments in each case numerous phases of sub vertical foliation development due to horizontal bulk shortening were followed by phases of gently dipping foliation formation due to gravitational collapse that accompanied thrusting during o00 and o1

use of paleo records in determining variability within the volcanism climate system abstract volcanic eruptions that inject large quantities of sulfur rich gases into the stratosphere have the capability of cooling global climate by 0 2 0 3 c for several years after the eruption equatorial eruptions will impact global climate whereas mid latitude eruptions can cool climate in the hemisphere of origin magnitude of cooling varies by latitude and it is possible for warming to occur in certain regions primarily during the winter although instrumental records have been used to quantify the volcanic impact on climate they are limited in their temporal and spatial coverage and the style and magnitude of eruptions occurring over the two centuries of instrumental records is limited a thorough evaluation of the range of variability in the volcanism climate system requires a multidisciplinary approach that includes the analysis of ice core records geological data atmospheric measurements and visual phenomena tree ring records and other proxy data evaluation of these longer time series indicates that multiple volcanic eruptions have the potential to force climate over decadal to multi decadal time frames especially when these eruptions enhance or extend pre existing cool conditions on the other hand a lack of climatically significant eruptions may result in warmer average temperatures over decadal time frames because the volcanic cooling component within the climate system is absent mega eruptions like the toba eruption of 71 000 73 000 yr ago may impact climate on centennial time frames through positive feedback mechanisms evidence exists which indicates that environmental changes associated with rapid climatic fluctuations such as crustal loading unloading with glaciation deglaciation and variability in glacial meltwater loading on ocean basins may cause an increase in volcanic activity

foliation intersection axes preserved in garnet porphyroblasts from the swat area nw himalaya a record of successive crustal shortening directions between the indian plate and kohistan ladakh island arc abstract we have investigated the geometry and tectonic significance of porphyroblast inclusion trails in 61 oriented samples from the swat region of north pakistan nw himalaya the studied samples represent the indian plate cover sequence deformed and metamorphosed to medium and high grade during the himalayan orogeny two complementary techniques were applied to measure the orientation of foliation intersection inflection axes fia in porphyroblasts in the studied samples and reveal the existence of three principal fia sets in the study area measuring approximately 60 30 km each fia set is characterized by a distinctive regionally consistent trend and relative timing from old to young ese wnw fia set1 e w fia set 2 and broadly nne ssw fia set 3 this sequence is inferred to record an anticlockwise rotation of the direction of tectonic transport or bulk crustal shortening perpendicular to fia through time either progressively or in a stepwise fashion consistent with previous kinematic and geochronological data regarding collisions between the indian plate kohistan ladakh island arc klia and eurasia between ca 55 and 40 ma fia set 1 and 2 correlate well with early formed ese wnw and e w trending thrust related structures in the study area respectively whereas the trend of fia 3 matches that of relatively young regional scale folds and shear zones with a conspicuous n s trend hence the fia succession has successfully resolved the relative timing of formation between the e w versus n s trending structures in the region which was a longstanding question in the tectonics of the nw himalaya in addition a comparison of orientation data for symmetrical millipede inclusion trails versus asymmetrical curved trails indicates that porphyroblasts experienced very limited differential rotations in the region despite the complex polyphase deformation history recorded perhaps the most fascinating aspect of our findings is the direct relationship that is suggested between fia trends and successive vectors of relative indian plate motion since collision with the klia

holocene sediments from a coastal lake on northern devon island nunavut canada sediment cores from lake dv09 northern devon island nunavut canada 75 34 34 n 89 18 55 w were studied to reconstruct the lake ontogeny through analysis and interpretation of the sediment stratigraphy the lake was uplifted from marine inundation 7600 cal bp after a millennium of rapid sediment accumulation which coincided with the holocene thermal maximum in the region accumulation rates decreased over the past 6000 years as the arctic became colder this resulted in the deposition of very fine laminae that were interpreted as varves the uppermost laminated sediments provided a 1600 year history of annual sediment transport and deposition into the lake during periods of warmer temperatures such as between 6000 and 7500 cal bp and during the medieval climate anomaly 950 1300 ce ce christian era hydroclimatic and permafrost slope processes increased sedimentation rates into the basin

ice cores from arctic sub polar glaciers chronology and post depositional processes deduced from radioactivity measurements the response of arctic ice masses to climate change is studied using ice cores containing information on past climatic and environmental features interpretation of this information requires accurate chronological data absolute dating of ice cores from sub polar arctic glaciers is possible using well known radioactive layers deposited by atmospheric nuclear tests maximum fallout in 1963 and the chernobyl accident 1986 analysis of several isotopes h 3 cs 137 shows that h 3 provides the most accurate dating of the 1963 maximum as indicated also in comparison with results from total beta measurements sr 90 and cs 137 mean annual net mass balances are derived from the dated ice cores from 1963 up to the date of the drillings the cs 137 and h 3 deposited by nuclear tests after decay correction are used to define a melt index for all 13 ice cores studied the relative strength of melting and percolation post depositional processes is studied on the basis of these cs 137 and h 3 deposits

a varve record of increased little ice age rainfall associated with volcanic activity arctic archipelago canada varved sediments from nicolay lake canadian high arctic record major summer rainfall events over the last five centuries increased incidences of summer rainfall occurred during the coldest periods of the little ice age and were strongly clustered in the years immediately following major volcanic events compari son of the summer rainfall and proxy air temperature records thus provides a fuller understanding of the nature and causes of natural climate variability in the arctic study of the synoptic conditions associated with the two most recent large summer rainfall events suggests that they are associated with the incursion of cold low pressure systems from the arctic ocean basin volcanic activity may produce atmospheric conditions more conducive to the formation of such low pressure systems which generate rainfall at low elevations and summer snowfall at higher elevations thus explaining the correlation between rainfall and summer snow accumulation recorded in ice cores from high elevation ice caps

holocene climatic records from agassiz ice cap ellesmere island nwt canada four ice cores from the top of the agassiz ice cap and down a flow line have been variously analysed for 8 18o ecm solid conductivity and ice melt layer stratigraphy stratigraphic correlation

suspended sediment transport in an estuarine tidal channel within san francisco bay california abstract a recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within san francisco bay california measurements were made every 15 min over six successive flood and ebb tidal cycles they included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed near bottom pressure fluctuations vertical temperature gradient and bottom photographs additionally suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle the instrument system was retrieved during each slack water period to exchange suspended sediment sample bags while the instrument was deployed std nephelometer measurements were made thoughout the water column and water samples were collected each 1 h and bottom sediment was sampled at the deployment site size distributions of the suspended sediment samples estimates of particle settling velocity s friction velocity u and reference concentration c a at z 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles three suspended sediment particle conditions were evaluated 1 individual particle sizes in the 4 11 62 5 0 5 m size range with the reference concentration c a at z 20 cm c 2 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration c a at z 20 cm c f and 3 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress an analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables s u and c a in addition computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions

episodic gravitational collapse and migration of the mountain chain during orogenic roll on in the himalayas an w e belt of maximum bulk horizontal shortening the orogen core moved north relative to the overlying crust to form the himalayan syntaxes due to roll on of this portion of the indian plate this displacement occurred below a lengthy succession of gently dipping decollements that formed episodically at a depth of 30 km along the orogen core due to numerous periods of gravitational collapse and spreading of the overlying ductile crust successively developed basal decollements were deformed when continued bulk horizontal shortening of the orogen core below reasserted dominance over the effects of gravitational collapse above causing refolding about steeply dipping axial planes this resulted in northwards migration of the orogen core above depths of 30 km causing rocks metamorphosing at depths of 22 km on the north side of the orogen core to be moved to its south side with no change in depth as roll on progressed garnet porphyroblasts record this lengthy history of lateral migration across the orogen within their inclusion trails the 6 4 kbar average pressures accompanying it were obtained from the mn fe and ca contents of successive garnet cores garnet grew at depths of 22 km until movement towards the surface initiated on successively developed decollements that accommodated the volume constraints of gravitational collapse and spreading on both sides of the orogen the speed of extrusional displacement increased the further the rocks migrated from the orogen core developing mylonitic schists around the porphyroblasts this truncated inclusion trails against all matrix foliations as the porphyroblasts were carried towards the surface indeed these rocks were multiply deformed during at least four distinct periods of deformation after mylonitization began and prior to exposure above the main central thrust mct three or more sub vertical and sub horizontal foliations were formed during each of the five changes in fia trend foliation inflection intersection axes in porphyroblasts preserved in these rocks the inclusion trail asymmetries and p t of garnet core growth accompanying each fia reveal that the first four changes in fia trend which define periods of tectonism about one direction of horizontal bulk shortening relative plate motion occurred on the north side of the orogen core the fifth occurred on the south side of the orogen core and the switch in shear sense on gently dipping foliation planes that resulted from this shift to the south eventually led to the development of the mct when magnetic anomaly 22 that formed in the southern indian ocean ridge is taken into account these five changes in fia trend correlate markedly with changes in the motion of india relative to a constant eurasia from 50 to c 25 ma they reveal that eurasia moved nnw during fias 1 3 and 4 and sse during fia 5 when the shear sense on gently dipping foliations switched to top to the s they suggest collision of india with eurasia took place at 50 ma immediately prior to the development of fia 1

the effects of meltwater percolation on the seasonal isotopic signals in an arctic snowpack we investigate the evolution of snow temperature water content density and stable water isotopes of 18 o at four arctic snow pit sites during early season melt in order to understand the effects of melt on snowpack stratigraphies and seasonal isotopic signals we relate isotopic changes observed at these sites to temperature reconstructions derived from a 33 year firn core record drilled on the same icefield decreases in seasonal isotopic amplitudes observed at all but one snow pit site coincide with the percolation of more enriched meltwater into the snowpack suggesting that meltwater percolation is the dominant process causing isotopic redistribution in arctic snowpacks during the melt season the decrease in isotopic range was accompanied by increases in mean 18 o values at all snow pit sites positive degree day pdd calculations are used to relate the amount of melt observed at the low elevation snow pit sites to the firn core site results based on pdd values suggest an average overestimation of 1 1 c in average annual temperature reconstructions from the firn core site from 1967 to 2006 with the possibility of errors in excess of 3 c during high melt years

correlating deformation and metamorphism around orogenic arcs the timing of metamorphism and the p t t path the rocks have undergone commonly varies around orogenic arcs hindering correlation along orogens isotopic dating enables broad correlations to be made but detailed correlations can be difficult using this approach where several phases of metamorphism are present this is compounded by the large size of oroclines which hinders correlation of deformation and metamorphism using a succession of structures and associated metamorphism however foliation inflexion intersection axes fias preserved in porphyroblasts provide a tool for correlation around oroclinal arcs consistent successions of fias that remain constantly oriented around folds and across large tracts of multiply ductilely deformed country rock allow correlation of periods of metamorphism taking place during the one direction of horizontal bulk shortening rather than correlation of inferred growth events consequently rocks that have undergone different p t t paths can be correlated measurement of fias around the spectacular kimberley arc in northwest australia reveals that the first formed set maintain a consistent average trend of 127 degrees and a subhorizontal plunge this fia trend is parallel to the western arm of the arc but lies at a very high angle to the eastern arm foliations preserved as inclusion trails in porphyroblasts are continuous with foliations in the matrix on the western arm where the fias and structural grain are sub parallel however inclusion trails in porphyroblasts defining fias with these same trends on the eastern arm are truncated by the matrix foliation this indicates that foliations in the matrix on the eastern arm postdate all foliations in the matrix on the western arm two late sets of locally developed small kinks of the matrix foliation on the western arm which have axial planes parallel to foliations on the eastern arm are weak expressions of the overprinting events that controlled the geometry of the latter arm a second group of fias has been found in some samples on the eastern arm that lie sub parallel to the northeast trend of this arm they are defined by inclusion trails that are continuous with the matrix foliation and that occur in the rims of porphyroblasts such porphyroblasts generally contain a core with inclusion trails defining the first formed fia that lies sub parallel to the western arm the inclusion trails in porphyroblast cores are truncated completely by those in the rims in the same manner that those without this rim set of inclusion trails are truncated by the matrix foliation this second set of fias has not been found in any samples from the western arm fias thus provide a simple tool for correlating metamorphism around orogenic arcs the porphyroblasts with consistently oriented fias around the arc can be readily correlated and grew during the same period of metamorphism that affected both arms those with northeast trending fia record a younger period of metamorphism that affected only the eastern arm indeed until techniques and standards for dating porphyroblasts and their inclusions by both the microprobe and ion microprobe improve fias may provide the only quantitative tool that integrates metamorphic and structural phenomena and thus allows detailed correlation of these events along an orogen

an inter hemispheric volcanic time marker in ice cores from greenland and antarctica a strong volcanic acid signal is clearly registered using an acidity measuring technique in the a d 1259 ice layer in four different greenland ice cores camp century milcent crete and dye 3 this signal is similar in amplitude to the laki iceland a d 1783 volcanic event as recorded in the central and south greenland ice cores measurement of ice layers from corresponding age levels in antarctic ice cores byrd station south pole and j 9 on the ross ice shelf provides similar strong acid signals there is no historical record of a significant volcanic eruption for the period around a d 1260 in the northern hemisphere subsequent chemical analyses of all a d 1259 ice layers show similar compositions we suggest that the a d 1259 signals registered in both greenland and antarctica were caused by the same volcanic disturbance and that its epicenter was located at the earth s equatorial zone which enabled global distribution of the acid gases these results indicate that inter hemispheric dating of ice sheets is possible by the chemical identification of major eruptive volcanic events in the equatorial zone

snow chemistry of agassiz ice cap ellesmere island northwest territories canada pit wall samples were collected from two sites about 2km apart on agassiz ice cap ellesmere island northwest territories canada in 1992 1993 and 1994 and from a site a further 1 km distant in order to study spatial and seasonal variations in snow chemistry two of the pits were dug in wind scoured zones and one in an unscoured zone although a large part of the winter snow is removed fromt he scoured zones which do not show very negarive 18 o values the winter spring anion peaks are still evident this may be due to the predominance of dry deposition in mid winter the cl and so 4 2 ions peak in later winter early spring while no 3 peaks both in later winter early spring and in summer vertical concentration profiles of all anions did not significantly alter over a 2 year period indicating that there are no serious post depositional changes due to evaporation snow melting or photochemical reactions however comparisons between stake board snow accumulation measurements and those derived from the least scoured pit indicate that a single pit will represent annual accumulation rates for a local area only

temperature time relationships across metamorphic zones evidence from porphyroblast matrix relationships in progressively deformed metapelites abstract in a rock pile undergoing prograde regional metamorphism the isotherms corresponding to certain metamorphic reactions shift continuously while the same rock volume develops progressive deformation fabrics as a result of increasing strain porphyroblasts in pelitic schists can provide crucial information on both these aspects of metamorphism not only do they reflect the pressure temperature evolution of that terrain but their inclusion trails are clues to the state of deformation at the time of porphyroblast growth microstructural relationships of inclusion trail patterns versus matrix foliations combined with petrological data on prograde reaction sequences can be used to reconstruct temperature time relationships in different zones within metamorphic terrains in two proterozoic areas in northern queensland the western mary kathleen fold belt and the robertson river subgroup a systematic variation of inclusion trail patterns has been found across metamorphic zones each porphyroblast species contains more advanced stages of s 2 foliation development towards its isograd compared with higher grade zones in which these porphyroblasts are present the combined microstructural petrographic observations allow to place constraints on the t t evolution of the various zones the regional distribution of inclusion trail patterns in the two study areas indicates that deformation affected high and low grade zones simultaneously

eight centuries of volcanic signal and climate change at talos dome east antarctica 1 during the 1996 programma nazionale di ricerche in antartide international trans antarctic scientific expedition traverse two firn cores were retrieved from the talos dome area east antarctica at elevations of 2316 m td 89 m long and 2246 m st556 19 m long cores were dated by using seasonal variations in non sea salt nss so42 concentrations coupled with the recognition of tritium marker level 1965 1966 and nss so42 spikes due to the most important volcanic events in the past pinatubo 1991 agung 1963 krakatoa 1883 tambora 1815 kuwae 1452 unknown 1259 the number of annual layers recognized in the td and st556 cores was 779 and 97 respectively the d record obtained from the td core has been compared with other east antarctic isotope ice core records dome c epica south pole taylor dome these records suggest cooler climate conditions between the middle of 16th and the beginning of 19th centuries which might be related to the little ice age lia cold period because of the high degree of geographical variability the strongest lia cooling was not temporally synchronous over east antarctica and the analyzed records do not provide a coherent picture for east antarctica the accumulation rate record presented for the td core shows a decrease during part of the lia followed by an increment of about 11 in accumulation during the 20th century at the st556 site the accumulation rate observed during the 20th century was quite stable

middle to late paleozoic acadian orogeny in the northern appalachians a laramide style plume modified orogeny the laramide orogeny of the western united states is proposed as a modern analogue for the silurian devonian acadian orogeny and subsequent diachronous voluminous short lived magmatism and basin formation in the northern appalachians shallowing of the benioff zone accounts for several enigmatic features associated with plate convergence in the northern appalachians including 1 wenlockian ludlovian termination of arc related magmatism in the avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 ma and 2 diachronous migration of the acadian deformation front from ca 415 ma in the southeast to ca 370 ma in the northwest extending more than 600 km into the continental interior the flattening of the subduction zone is attributed to overriding of a plume by the convergent margin which may explain 1 the abrupt termination of magmatic quiescence by 380 370 ma and voluminous felsic magmatism and production of plume related lamprophyres in the southeast meguma terrane as the plume thermally eroded the oceanic lithosphere causing melting of the lower crust 2 late devonian regional high temperature low pressure metamorphism in the meguma terrane related to the thermal anomalies above a plume 3 synchronous devonian emplacement of meguma gold deposits and associated siderophile elements possibly derived from fluid circulation above an ascending plume 4 rapid late devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume 5 migration of magmatism to the north avalon terrane cobequid highlands so that plume related carboniferous magmatism occurred in and around the carboniferous permian maritimes basin 6 the high density lens at the base of the crust beneath the maritimes basin the product of plume derived underplated mafic rocks and 7 a subsidence mechanism for formation of the maritimes basin by cooling of a decapitated plume head

porphyroblast rotation during crenulation cleavage development an example from the aureole of the mooselookmeguntic pluton maine usa in the low pressure high temperature metamorphic rocks of western maine usa staurolite porphyroblasts grew at c 400 ma very late during the regional orogenesis these porphyroblasts which preserve straight inclusion trails with small thin section scale variation in pitch were subsequently involved in the strain and metamorphic aureole of the c 370 ma mooselookmeguntic pluton the aureole shows a progressive fabric intensity gradient from effectively zero emplacement related deformation at the outer edge of the aureole 2900 m map distance from the pluton margin to the development of a pervasive emplacement related foliation adjacent to the pluton the development of this pervasive foliation spanned all stages of crenulation cleavage development which are preserved at different distances from the pluton the spread of inclusion trail pitches in the staurolite porphyroblasts as measured in two dimensional 2 d thin sections increases nonlinearly from 16 to 75 with increasing strain in the aureole these data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage the data spread is qualitatively modelled for both pure and simple shear and both solutions match the data reasonably well the spread of inclusion trail orientations 40 75 in the moderately to highly strained rocks is similar to the spread reported in several previous studies we consider it likely that the sample scale spread in these previous studies is also the result of porphyroblast rotation relative to one another however the average inclusion trail orientation for a single sample may in at least some instances reflect the original orientation of the overgrown foliation

pseudomorphs and associated microstructures of western maine usa abstract in metapelitic rocks of western maine a pluton related m3 metamorphic gradient ranging in grade from garnet to upper sillimanite zone was superposed on a fairly uniform m2 regional metamorphic terrain characterized by the assemblage andalusite staurolite biotite garnet as a result m2 assemblages re equilibrated to the p t and ah2o conditions of m3 and both prograde and retrograde pseudomorphism of m2 porphyroblasts occurred the type of pseudomorph and degree of development is directly related to the rock s position within the m3 metamorphic gradient a function of its proximity to the mooselookmeguntic pluton several hinge zones occur in which the m3 minerals that pseudomorphed a particular m2 phase change for example m2 garnet was replaced by m3 chlorite or biotite depending on its position within the m3 gradient similarly in a transition zone between m3 upper staurolite and lower sillimanite zones m2 staurolite was stable and shows m3 growth rims downgrade from this transition zone staurolite was pseudomorphed by chlorite and muscovite whereas upgrade the pseudomorphs contain muscovite and some biotite m3 pseudomorphs commonly retain crystal shapes of the original m2 porphyroblasts reflecting relatively low regional deviatoric stress during and after m3 although evidence for textural disequilibrium is common chemical equilibrium was closely approached during m3 this study demonstrates for m3 that 1 the pseudomorphic replacement was a constant volume process and 2 fabrics produced by tectonic events can be erased by subsequent deformation and or sufficiently intense subsequent recrystallization

lateglacial and holocene climate disturbance and permafrost peatland dynamics on the seward peninsula western alaska abstract northern peatlands have accumulated large carbon c stocks acting as a long term atmospheric c sink since the last deglaciation how these c rich ecosystems will respond to future climate change however is still poorly understood furthermore many northern peatlands exist in regions underlain by permafrost adding to the challenge of projecting c balance under changing climate and permafrost dynamics in this study we used a paleoecological approach to examine the effect of past climates and local disturbances on vegetation and c accumulation at a peatland complex on the southern seward peninsula alaska over the past 15 ka 1 ka 1000 cal yr bp we analyzed two cores about 30 m apart nl10 1 from a permafrost peat plateau and nl10 2 from an adjacent thermokarst collapse scar bog for peat organic matter om c accumulation rates macrofossil pollen and grain size analysis a wet rich fen occurred during the initial stages of peatland development at the thermokarst site nl10 2 the presence of tree pollen from picea spp and larix laricinia at 13 5 12 1 ka indicates a warm regional climate corresponding with the well documented bolling allerod warm period a cold and dry climate interval at 12 1 11 1 ka is indicated by the disappearance of tree pollen and increase in poaceae pollen and an increase in woody material likely representing a local expression of the younger dryas yd event following the yd the warm holocene thermal maximum htm is characterized by the presence of populus pollen while the presence of sphagnum spp and increased c accumulation rates suggest high peatland productivity under a warm climate toward the end of the htm and throughout the mid holocene a wet climate induced several major flooding disturbance events at 10 ka 8 1 ka 6 ka 5 4 ka and 4 7 ka as evidenced by decreases in om and increases in coarse sand abundance and aquatic fossils algae chara and water fleas daphnia the initial peatland at permafrost site nl10 1 is characterized by rapid c accumulation 66 g c m 2 yr 1 high om content and a peak in sphagnum spp at 5 8 4 6 ka suggesting the lack of permafrost a transition to extremely low c accumulation rates of 6 3 g c m 2 yr 1 after 4 5 ka at this site suggests the onset of permafrost aggradation likely in response to neoglacial climate cooling as documented across the circum arctic region a similar decrease in c accumulation rates also occurred at non permafrost site nl10 2 time weighted c accumulation rates are 21 8 g c m 2 yr 1 for core nl10 1 during the last 6 5 ka and 14 8 g c m 2 yr 1 for core nl10 2 during the last 15 ka evidence from peat core analysis and historical aerial photographs shows an abrupt increase in sphagnum spp and decrease in area of thermokarst lakes over the last century suggesting major changes in hydrology and ecosystem structure likely due to recent climate warming our results show that the thermokarst permafrost complex was much more dynamic with high c accumulation rates under warmer climates in the past while permafrost was stabilized and c accumulation slowed down following the neoglacial cooling in the late holocene furthermore permafrost presence at local scales is controlled by both regional climate and site specific factors highlighting the challenge in projecting responses of permafrost peatlands and their c dynamics to future climate change

solid electrical conductivity ecm from four agassiz ice cores ellesmere island nwt canada high resolution signal and noise over the last millennium and low resolution over the holocene the solid dc electrical conductivity ecm electrical conductivity method of polar ice cores has become an important tool in identifying and quantifying volcanic acid layers and this paper addresses the question of how much signal and noise there is in single ecm series a number of high resolution 10 samples year ice core ecm records from the agassiz ice cap are correlated over the last 900 years corre lations decrease with distance apart due to local drift and melt layer noise but correlations are probably reduced also by differences in methodology and core storage it is found that only peak sizes in the uppermost two percentiles retain their ranking and recognizability from core to core with continuous sampling however the smaller peaks can be cross identified between cores even though they lose their size rank averaging or stacking several ecm records reduces the noise five year averages of ecm for the holocene are presented for the agassiz cores and their correlations interpreted as functions of distance apart and differences in method the large scale melting in the early holocene 8ka to 10ka almost completely de acidifies the ice in all the agas siz cores

greenland accumulation an error model 1 total surface accumulation total snowfall minus total sublimation on the greenland ice sheet is estimated as 299 23 kg m 2 yr 1 for an assumed 30 year span the uncertainty being quoted as twice the standard error the estimate is very similar to earlier estimates because it relies largely on the same compiled observations but it is the first to be accompanied by formal error bars an error model was developed for this purpose it incorporates uncertainties due to measurement error record length or span and spatial bias in the distribution and density of the observations measurement errors although dominant over large parts of the interior make only a small contribution to uncertainty in the accumulation of the whole ice sheet time series of accumulation are shown to be stationary and independent which means that the date of any given observation is of no importance when evaluating its uncertainty as an estimate of accumulation at any other date however as found earlier the span of the observation is a leading contributor to uncertainty and a means of placing short and long span measurements on an equal footing is developed spatial bias is the other leading contributor to judge from modeling of the standard error of averages of pairs of observations as a function of their separation and from a map of the standard error of accumulation the standard error of interpolated point estimates of accumulation is small where measurements are dense and well distributed in the interior but grows rapidly where measurements are more thinly or unevenly distributed estimates of accumulation are therefore most uncertain in the peripheral ablation zone it is suggested that uncertainty in accumulation needs to be reduced by a factor of two or more if the measurements are to contribute reliably to explaining the observed sea level rise

the 1452 or 1453 a d kuwae eruption signal derived from multiple ice core records greatest volcanic sulfate event of the past 700 years 1 we combined 33 ice core records 13 from the northern hemisphere and 20 from the southern hemisphere to determine the timing and magnitude of the great kuwae eruption in the mid 15th century we extracted volcanic deposition signals by applying a high pass loess filter to the time series and examining peaks that exceed twice the 31 year running median absolute deviation by accounting for the dating uncertainties associated with each record these ice core records together reveal a large volcanogenic acid deposition event during 1453 1457 a d the results suggest only one major stratospheric injection from the kuwae eruption and confirm previous findings that the kuwae eruption took place in late 1452 or early 1453 which may serve as a reference to evaluate and improve the dating of ice core records the average total sulfate deposition from the kuwae eruption was 93 kg so4 km 2 in antarctica and 25 kg so4 km 2 in greenland the deposition in greenland was probably underestimated since it was the average value of only two northern greenland sites with very low accumulation rates after taking the spatial variation into consideration the average kuwae deposition in greenland was estimated to be 45 kg so4 km 2 by applying the same technique to the other major eruptions of the past 700 years our result suggests that the kuwae eruption was the largest stratospheric sulfate event of that period probably surpassing the total sulfate deposition of the tambora eruption of 1815 which produced 59 kg so4 km 2 in antarctica and 50 kg so4 km 2 in greenland

monazite ages in the chesham pond nappe sw new hampshire u s a implications for assembly of central new england thrust sheets four distinct generations of monazite growth have been identified in samples from the chesham pond nappe and three monazite compositional domains 2 3 and 4 have been correlated with both temperature and mineral assemblage domain 1 cores were interpreted previously to be detrital relics or vestiges of an earlier acadian metamorphism the four monazite domains have been dated by in situ isotope and chemical methods the following are chemical ages of each domain weighted average 2 standard errors of the mean 400 10 ma domain 1 381 8 ma domain 2 372 6 ma domain 3 352 14 ma domain 4 heating and cooling rates derived from combining monazite ages monazite thermometry and 40ar 39ar closure temperatures are approximately 10 15 c m y for heating from 470 to 740 c approximately 8 c m y for cooling from 740 to 375 c and approximately 1 2 c m y for cooling from 375 to 150 c temperature time paths calculated with monazite ages and monazite thermometry indicate that 1 plutonism at ca 400 ma was the likely heat source for the formation of monazite domain 1 and 2 monazite domains 2 4 were produced during a regional low pressure high temperature metamorphism active between 380 350 ma the regional metamorphism is ascribed to lithospheric mantle delamination followed by asthenospheric mantle upwelling which heated a wide area of the merrimack basin southwestern new hampshire central massachusetts central connecticut to temperatures in excess of 725 c monazite ages in the chesham pond nappe and adjacent structural units to the west constrain the commencement of nappe overthrusting to roughly 355 ma