age geochemistry and sm nd isotopic signature of the 0 76 ga burin group compositional equivalent of avalonian basement abstract the ca 760 ma burin group occurs in a well exposed 60 km long northeasterly trending belt on the burin peninsula in the avalon zone of the southeastern newfoundland appalachians this lithologically distinctive tectonically bounded succession is characterized by low grade massive and pillowed basalts abundant gabbro and diabase dykes and sills mafic pyroclastic and epiclastic rocks and stromatolitic carbonate bearing rocks the burin group preserves the oldest known magmatic event in the avalon terrane but its tectonic evolution and its relationship to the voluminous 640 570 ma arc related magmatism that typifies avalonia is uncertain geochemical analyses confirm that the basalts are predominantly low k tholeiites they are characterized by high ratios of lil hfs elements and display depletion to slight enrichment in lree sm nd isotopic data reveal that most basalts have juvenile compositions with nd values similar to contemporaneous depleted mantle indicating that high lil hfs elemental ratios were probably due to coeval subduction which contaminated the mantle source other basalts have lower nd values and the negative correlation of nd with la sm together with a positive correlation of nd with 147 sm 144 nd suggest that their isotopic signatures have been modified by a mesoproterozoic older oceanic crust or mantle into which the burin group mafic magmas were emplaced the isotopic signature of the burin mafic rocks is similar to that inferred for the source of the main phase of avalonian magmatism these data together with paleocontinental reconstructions for ca 760 ma suggest that the burin group is a local representative of ensimatic arcs within the panthalassa type ocean that surrounded rodinia possibly as a far field response to the breakup of rodinia although its low metamorphic grade precludes it being the basement from which avalonian magmas were extracted the burin group may be representative of the geochemical and isotopic composition of that basement

proto avalonia a 1 2 1 0 ga tectonothermal event and constraints for the evolution of rodinia the neoproterozoic evolution of avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of rodinia similar sm nd isotopic signatures for different periods of arc activity suggest that avalonian basement or proto avalonia was generated in a series of primitive oceanic island arcs between 1 2 and 1 0 ga because this interval coincides with the amalgamation of rodinia proto avalonia is inferred to have been located in a panthalassa like peri rodinian ocean an early 760 660 ma phase of avalonian arc activity is attributed to renewed subduction in the peri rodinian ocean following the breakup of rodinia which caused the accretion of avalonian terranes to the gondwanan margin by ca 650 ma further subduction along the margin occurred outboard of these terranes and resulted in the onset of main phase avalonian volcanism at 630 ma the diachronous cessation of arc magmatism is attributed to ridge trench collision and the generation of a continental transform the geodynamic link between avalonia and rodinia is analogous to that between the mesozoic dispersal of pangea and the tectonothermal evolution of western north america this event also resulted in the accretion of outboard terranes and in arc related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the san andreas transform the model implies that the neoproterozoic evolution of avalonia and other peri gondwanan terranes provide important constraints on the tectonic history of a large portion of the rodinian continental margin

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

detrital zircon characterization of early cambrian sandstones from east avalonia and se ireland implications for terrane affinities in the peri gondwanan caledonidesj w f waldron and othersdetrital zircon peri gondwanan caledonides the caledonides of britain and ireland include terranes attributed to both laurentian and gondwanan sources separated along the solway line gondwanan elements to the south have been variably assigned to the domains ganderia and east avalonia the midland platform forms the core of east avalonia but its provenance is poorly known laser ablation split stream analysis yields information about detrital zircon provenance by providing simultaneous u pb and lu hf data from the same ablated volume a sample of red callavia sandstone from uppermost cambrian stage 3 of the midland platform yields a u pb age spectrum dominated by neoproterozoic and palaeoproterozoic sources resembling those in the welsh basin the meguma terrane of nova scotia and nw africa initial ehf values suggest that the neoproterozoic zircon component was derived mainly from crustal sources

ash layers from iceland in the greenland grip ice core correlated with oceanic and land sediments four previously known ash layers ash zones i and ii saksunarvatn and the settlement layer all originating in iceland have been identified in the central greenland ice core grip this correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash this establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately a precise connection with dates obtained by14c beyond the range of dendrochronology is established which provides an excellent confirmation of230th 234u dates from corals four additional icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits

a comparative analysis of pre silurian crustal building blocks of the northern and the southern appalachian orogen the new york promontory serves as the divide between the northern and southern segments of the appalachian orogen antiquated subdivisions distinct for each segment implied that they had lithotectonic histories that were independent of each other using new lithotectonic subdivisions we compare first order features of the pre silurian orogenic building blocks in order to test the validity of the implication of independent lithotectonic histories for the two segments three lithotectonic divisions termed here the laurentian iapetan and the peri gondwanan realms characterize the entire orogen the laurentian realm composed of native north american rocks is remarkably uniform for the length of the orogen it records the multistage neoproterozoic early paleozoic rift drift history of the appalachian passive margin formation of a taconic seaway and the ultimate demise of both in the middle ordovician the iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the iapetus ocean between laurentia and gondwana in the northern segment the realm is divisible on the basis of stratigraphy and faunal provinciality into peri laurentian and peri gondwanan tracts that were amalgamated in the late ordovician south of new york stratigraphic and faunal controls decrease markedly rock associations are not inconsistent with those of the northern appalachians although second order differences exist exposed exotic crustal blocks of the peri gondwanan realm include ganderia avalonia and meguma in the north and carolinia in the south carolinia most closely resembles ganderia both in early evolution and late ordovician silurian docking to laurentia our comparison indicates that to a first order the pre silurian appalachian orogen developed uniformly starting with complex rifting and a subsequent drift phase to form the appalachian margin followed by the consolidation of iapetan components and ending with accretion of the peri gonwanan ganderia and carolinia this deduction implies that any first order differences between northern and southern segments post date late ordovician consolidation of a large portion of the orogen

relative changes of the geomagnetic field intensity during the last 280 kyear from piston cores in the acores area abstract relative changes in geomagnetic field intensity over the last 280 kyears have been recovered from the study of three marine cores from the acores area one core was precisely dated by oxygen isotope study and the other two records were linked to it using light reflectance analysis which allowed precise correlation rock magnetic analysis shows that the main magnetic mineral is magnetite with a very homogeneous grain size distribution in the pseudo single domain range for the three cores changes in the amount of magnetite do not exceed a factor of 10 therefore these cores appear to be suitable for relative palaeointensity determinations two mineral magnetic components with periodicities of 23 and 18 kyears are present in the records of bulk magnetic parameters and natural remanent magnetisation nrm but the power at these frequencies is not significant at the 95 level for the normalised remanence records for each core normalisation of the nrm using different normalising parameters yields virtually identical results using saturation isothermal remanent magnetisation sirm as the normalising parameter the results of the three cores were then combined into a stacked curve squared coherence analysis between this stacked curve and bulk mineral magnetic parameters reveals that the 18 kyear component is still present in the record but that the power spectrum is barely above noise level at this frequency some of the features of this north atlantic record are consistent with the main characteristics already documented in other sedimentary or volcanic records for instance distinct periods of low intensity are observed around 40 120 and 190 kyears and periods of high intensity at 50 and 80 kyears because of the uniformity of the mineral magnetic characteristics of the three cores we suggest that this record may be a suitable palaeointensity reference curve for the central north atlantic ocean region

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10be in globally stacked deep sea sediments the reconstruction of geomagnetic field intensity variations during the last 200 kyr from paleomagnetic data is at present the subject of numerous studies and major debate there is currently no generally accepted record here we present a global stacked record of 230thex normalized 10be deposition in marine sediments representing relative variations in 10be production rate which are translated into field intensity variations the record shows major periods during which the field intensity was between 10 and 40 of the present day value namely 30 42 60 75 85 110 and 180 192 kyr b p our results are compared to independently derived paleomagnetic studies and th u calibrations of 14c dates on corals during most of the observed period the geomagnetic field intensity was weaker than today resulting in an overall 30 reduced value for the last 200 kyr

the importance of late and post orogenic crustal growth in the early proterozoic evidence from smnd isotopic studies of igneous rocks in the makkovik province canada abstract sm nd isotopic studies suggest that large tracts of 1900 1700 ma old crust in laurentia baltica is of juvenile origin this crust has generally been ascribed to arc magmatism sustained over long periods and most conceptual models for crustal growth emphasize this process the late to post orogenic granitoid rocks that areally dominate many ancient orogenic belts are commonly viewed as anatectic derivatives of the earlier arc type crust however in regions of short crustal residence the time resolution of isotopic tracers such as the sm nd system does not permit discrimination between this model and continued growth of the crust during post orogenic magmatism the relative contributions of recycled crust and new mantle derived material in late to post orogenic magmas can only be assessed where they also transect much older crustal blocks the nd isotopic signatures of 1800 1720 ma igneous suites in the makkovik province define such a boundary between an archean craton and a juvenile proterozoic domain in the juvenile domain the nd signatures of most igneous suites are equivocal initial nd 0 to 2 and they could be anatectic derivatives of slightly older orthogneisses however addition of new mantle derived material is documented by a type granites with initial nd up to 4 in the cratonic domain temporally and compositionally equivalent igneous suites mostly have initial nd of 7 to 3 significantly above local archean basement which has nd of 15 at 1800 ma conservative calculations suggest that most of these suites contain more than 50 new mantle derived material these results suggest significant crustal growth via late stage magmatism and direct interaction of mantle derived magmas and lower crustal rocks in contrast 1650 ma igneous suites lack clear systematic variation in nd and are interpreted as representing crustal growth via later distal arc type magmatism with crust mantle mixing mainly facilitated by sediment subduction crustal growth in this area thus involved significant addition of new material through late to post orogenic magmatism this process may partially explain the unusually high growth rates calculated for some segments of proterozoic crust which approach or exceed modern global arc addition rates this stage 3 crustal growth may have been proportionally more important in early earth history due to higher ambient mantle temperatures

silurian u pb zircon intrusive ages for the red river anorthosite northern cape breton island implications for the laurentia avalonia boundary in atlantic canada abstract current interpretations of the geology of cape breton island suggest that it exposes a complete cross section of the appalachians from laurentia across iapetan vestiges to avalonia crucial to this view is the presence of ca 1 ga plutons including anorthosites which have been regarded as correlatives of grenvillian basement a correlation that overlooks the fact that avalonia is also underlain by a ca 1 ga basement we analyzed zircons from the red river anorthosite blair river complex northwestern cape breton island previously dated as ca 1 1 ga they yielded 421 3 ma intrusive ages with older ages between 865 18 ma and 1044 20 ma inferred to be either xenocrysts derived from the country rock or from the source implications of these data suggest that the accompanying low pressure granulite amphibolite facies metamorphism of the blair river complex is either the root of a 440 410 ma magmatic belt produced during slab break off or relict ca 1 ga basement the blair river complex occurs in a nne ssw sinistral positive flower structure that progresses upwards from a neoproterozoic rifted arc through a low grade upper ordovician silurian overstep sequence to amphibolite facies fault slices capped by the low pressure granulite facies rocks blair river complex the correlation of neoproterozoic rifted arc units across most of cape breton island suggests it represents the deformed northwestern margin of avalonia intruded by a silurian lower devonian magmatic belt as the geological record in the blair river complex is similar to both grenvillian and avalonian basements its provenance is equivocal however pb isotopic data suggest the blair river complex has amazonian avalonia affinities thus cape breton island rather than representing a complete cross section of the appalachian orogen is part of pristine deformed avalonia with a positive flower structure exposing a cross section of avalonian crust

ams radiocarbon dating and varve chronology of lake soppensee 6000 to 12000 14c years bp for the extension of the radiocarbon calibration curve beyond 10000 14c y bp laminated sediment from lake soppensee central switzerland was dated the radiocarbon time scale was obtained using accelerator mass spectrometry ams dating of terrestrial macrofossils selected from the soppensee sediment because of an unlaminated sediment section during the younger dryas 10000 11000 14c y bp the absolute time scale based on counting annual layers varves had to be corrected for missing varves the soppensee radiocarbon verve chronology covers the time period from 6000 to 12000 14c y bp on the radiocarbon time scale and 7000 to 13000 calendar y bp on the absolute time scale the good agreement with the tree ring curve in the interval from 7000 to 11450 cal y bp cal y indicates calendar year proves the annual character of the laminations the ash layer of the vasset killian tephra massif central france is dated at 8230 140 14c y bp and 9407 44 cal y bp the boundaries of the younger dryas biozone are placed at 10986 69 cal y bp younger dryas preboreal and 1212 86 cal y bp allerod younger dryas on the absolute time scale the absolute age of the laacher see tephra layer dated with the radiocarbon method at 10 800 to 11200 14c y bp is estimated at 12350 135 cal y bp the oldest radiocarbon age of 14190 120 14c y bp was obtained on macrofossils of pioneer vegetation which were found in the lowermost part of the sediment profile for the late glacial the offset between the radiocarbon 10000 12000 14c y bp and the absolute time scale 11400 13000 cal y bp in the soppensee chronology is not greater than 1000 years which differs from the trend of the u th radiocarbon curve derived from corals

geomagnetic secular variation as recorded in british lake sediments and its application to archaeomagnetic studies abstract lake sediments can play an important role in understanding and reconstructing temporal characteristics of the geomagnetic field as they potentially offer near continuous high resolution archives of magnetic information extending throughout the holocene to date lake sediment geomagnetic data has typically been excluded from british archaeomagnetic secular variation curves svc due to uncertainties with the acquisition of magnetisation by lake sediments this paper presents the argument that with regards to british datasets the real problem lies with poor chronological control and sets out to illustrate that with british archaeomagnetic data some progress has been made the results indicate that it is not currently possible to resolve secular variation on a time scale of 100 years from published british lake sediment data but more success has been made with data from archaeological sediments this level of detail has been considered necessary for the incorporation of lake sediment data into the british archaeomagnetic dataset as the ability to resolve short term geomagnetic changes is critical for the integration of any dataset into the british svc as the latter is predominantly employed to date archaeological architecture and artefacts requires that it has the ability to resolve changes over timescales relevant to human lifetimes using currently available data this retrospective critique confirms that in archaeological sediments depositional and thermoremanent magnetisation can record the same direction over the same time interval

electrical conductivity measurements from the gisp2 and grip greenland ice cores the direct current electrical conductivity of glacial ice depends on its acidity1 3 and can also indicate changes in climate as ice formed in cold dusty periods has a high concentration of alkaline dust1 4 5 which significantly reduces the conductivity6 7 compared to warmer less dusty periods here we present electrical conductivity records for the greenland ice sheet project 2 gisp2 and greenland ice core project grip ice cores drilled 28 km apart to enable direct comparison of the results the upper parts of both records are consistent with previous evidence from other greenland cores4 8 12 for a stable greenland climate during the holocene and a series of warm events punctuating the last glacial period however there is a significant discrepancy between the two records in the bottom 10 of the cores calling into question recent reports of climate variability in the last interglacial4 8 and the penultimate glaciation8 at this stage it is too early to say what exactly is causing the discrepancy although ice flow may have introduced some discontinuities into the records further work will be necessary to establish how much climatic information it will eventually be possible to extract from the lower parts of the two cores

tonian ediacaran tectonomagmatic evolution of west avalonia and its ediacaran early cambrian interactions with ganderia an example of complex terrane transfer due to arc arc collision

depositional remanent magnetization toward an improved theoretical and experimental foundation the first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field yet redeposition experiments showed a strong and quasi linear dependence on the external field this empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades we present redeposition experiments which suggest instead that the relationship between depositional remanence drm and applied field may frequently be curved for magnetic fields in the range of the earth s numerical simulations using a flocculation model can explain the redeposition data and suggest that drm will be significantly non linear when the flocs are small several microns there is a strong dependence of floc size on salinity particularly in low salinity environments floc size has a profound influence on the efficiency of drm hence low salinity environment may give results with poor reproducibility the size of the floc in which magnetic particles are embedded is not accounted for in current methods of normalization yet is the most important parameter on the bright side however it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floc size distribution and settling times can be constrained

radiolitid rudist colonisation strategies and biostrome development in moderate energy inner platform environments campanian brac island croatia abstract during the late cretaceous radiolitid rudist bivalves were abundant calcifiers on tethyan shallow water carbonate platforms previous studies have demonstrated various low to high energetic subtidal environments in which vertically growing rudist bivalves lived however the precise mode of colonisation in moderate to higher energetic environments remains unclear the purpose of this study is to elucidate the attachment of rudist juveniles and colonisation strategies on loose grain sediment using three dimensional reconstructions and other quantitative approaches the present data demonstrate that small and thinly walled rudists colonised energetic environments over several generations the main contributors to the biostrome distefanella bournonia and pseudopolyconites followed different strategies within the same ecosystem for instance cylindrical elevator morphotype distefanella built up a generally loose but anchored framework to resist sedimentation and wave energy in contrast bournonia and pseudopolyconites followed a solitary strategy of settlement on any hard substrate preferred growth direction affecting all genera was possibly linked to a predominant direction of water currents

origin and evolution of avalonia evidence from u pb and lu hf isotopes in zircon from the mira terrane canada and the stavelot venn massif belgium laser ablation inductively coupled plasma mass spectrometry u pb and lu hf isotope analyses of detrital zircon from neoproterozoic cambrian clastic sedimentary rocks in the mira terrane cape breton island nova scotia canada west avalonia and the stavelot venn massif east belgium east avalonia support deposition on an originally coherent microcontinent crustal evolution trends defined by hf t values varying with age reflect juvenile magma production in the source continent at 1 2 2 2 and 2 4 3 1 ga mixing of juvenile and recycled crust in continental magmatic arcs is recognized at 0 5 0 72 1 4 1 7 1 8 2 2 and 2 4 2 7 ga these results concur with the crustal evolution in amazonia the likely parent craton crustal evolution in avalonia is recorded in detrital and magmatic zircon from neoproterozoic arcs 680 550 ma positive hf t values suggest juvenile input and mixing with recycled crust most negative hf t values represent recycling of predominantly mesoproterozoic underlying crust avalonian arc magmatism was followed by late neoproterozoic early cambrian sedimentation in various belts in west avalonia these belts were juxtaposed by strike slip during late early cambrian deposition in a rift basin the youngest detrital zircon population c 517 ma probably represents synrift magmatism before break up of avalonia migmatization at 406 2 ma in a xenolith from the east avalonian crust reflects post collisional heating supplementary materials details of sample locations and analytical results are available at www geolsoc org uk sup18641

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

beryllium 10 in the greenland ice core project ice core at summit greenland concentrations of the cosmogenic isotope 10be have been measured in more than 1350 samples from the greenland ice core project grip ice core drilled at summit greenland although a dust associated component of 10be retained by 0 45 m filters in some of the samples complicates the interpretations the results confirm that the first order origin of 10be concentration variations is changes in precipitation rate associated with different climate regimes this effect is seen not only between glacial and interglacial periods but also during the shorter dansgaard oeschger interstadials by contrast the 10be data do not support the interpretation of rapidly varying accumulation i e climate during the last interglacial they can however be used to help place limits on the origin of the ice in these events after taking into account variable snow accumulation effects variations in the 10be flux are observed probably caused by solar and geomagnetic modulation but possibly also by primary cosmic ray variations the most dramatic is a 10be peak 40 000 years ago similar to that found in the vostok ice core thus permitting a very precise correlation between climate records from arctic and antarctic ice cores the 36cl 10be ratio considering either total or only ice associated 10be shows significant variability over the whole core depth thus confirming the difficulty in using this parameter for dating ice cores

petrogenesis and paleotectonic history of the wild bight group an ordovician rifted island arc in central newfoundland the wild bight group wbg is a sequence of early and middle ordovician volcanic subvolcanic and epiclastic rocks part of the dunnage tectonostratigraphic zone of the newfoundland appalachians a detailed geochemical and nd isotopic study of the volcanic and subvolcanic rocks has been carried out to determine the geochemical characteristics of the rocks interpret their palcotectonic environments and constrain their petrogenetic history the lower and central stratigraphic levels of the wbg contain mafic volcanic rocks with island arc geochemical signatures including lree enriched are tholeiites with e nd t 0 1 to 2 2 type a i lree depleted arc tholeiites with e nd t 5 6 to 7 1 type a ii and an unusual suite of strongly incompatible element depleted tholeiites in which e nd t ranges from 0 9 to 4 6 and is negatively correlated with147sm 144nd type a iii high silica low k rhyolites occur locally in the central part of the stratigraphy associated with mafic rocks of arc affinity and have e nd t 4 7 to 5 4 the upper stratigraphic levels of the wbg dominantly contain rocks with non arc geochemical signatures including alkalic basalts with e nd t 4 6 to 5 5 type n i strongly lree and incompatible element enriched tholeiites that are transitional between alkalic and non alkalic rocks with e nd t 4 4 to 7 0 type n ii and rocks with flat to slightly lree enriched patterns and e nd t 5 1 to 7 4 type n iii rocks with non arc and arc signatures are locally interbedded near the stratigraphic type of the wbg nd isotopic data in the type a i and a ii rocks are generally compatible with mixing partial melting models involving depleted mantle variably contaminated by a subducted crustally derived sediment the petrogenesis of type a iii rocks must involve source mixing and multi stage partial melting but the details are not clear the geochemistry and nd isotope data for types n i n ii and n iii rocks are compatible with petrogenetic models involving variable partial melting of a source similar to that postulated for modern oceanic island basalts comparison of the wbg with modern analogues suggests a 3 stage developmental model stage 1 island arc volcanism eruption of type mafic volcancs stage 2 arc rifting continued eruption of type a i a i eruption of types a ii and a iii mafic volcanics and high silica low k rhyolites and stage 3 back arc basin volcanism continued minor eruption of type a i basalts eruption of types n i n ii n iii basalts stages 1 and 2 volcanism involved partial melting of subduction contaminated mantle while stage 3 volcanism utilized depleted mantle sources not affected by the subducting slab this model provides a basis for interpreting coeval sequences in central newfoundland and a comparative framework for some early paleozoic oceanic volcanic sequences elsewhere in the appalachian orogen

lu hf zircon and sm nd whole rock isotope constraints on the extent of juvenile arc crust in avalonia examples from newfoundland and nova scotia canada avalonia the largest accreted crustal block in the appalachian orogen consists of neoproterozoic magmatic arc sequences that represent protracted and episodic subduction related magmatism before deposition of an ediacaran ordovician cover sequence including siliciclastic rocks zircon crystals were obtained from arc related magmatic rocks and from clastic sedimentary sequences and analyzed in situ for their hf isotope composition the majority of magmatic and detrital zircons are dominated by initial 176hf 177hf values that are more radiogenic than chondritic uniform reservoir chur with calculated crust formation hf tdm model ages that range from 0 84 to 1 30 ga these results suggest formation by partial melting of juvenile mantle in a neoproterozoic continental arc some zircons have hf tdm model ages ca 1 39 3 09 ga with ehf values of 33 9 to 0 5 and more clearly indicate involvement of older lithosphere in their petrogenesis whole rock sm nd isotopic compositions from felsic volcanic rocks are

the extent of juvenile crust in the grenville province nd isotope evidence a large nd isotope data set is presented to generate a crustal formation age map for the southwest grenville province placing it in the context of the province as a whole previous studies have shown that most crust in the central and eastern parts of the province is pre grenvillian composed largely of juvenile 1 5 1 7 1 9 and 2 7 ga arc terranes partly reworked by later ensialic arcs and collision zones in contrast new nd data reported here show that juvenile monocyclic crust is restricted to the sw grenville province in part of the central metasedimentary belt interpreted as a failed backarc rift zone there is a clear isotopic distinction between this rift zone and the pre grenvillian crust on the flanks on the flanks of the rift zone there is evidence for two crust forming events comprising an earlier 1 5 ga juvenile pinwarian event and a later 1 4 1 3 ga continental margin arc the elzevirian orogeny based on the geographical distribution of nd model ages in the sw grenville province a revised tectonic model for the grenville orogeny is developed in this model backarc spreading created a rift zone behind a pre grenvillian elzevirian subduction zone that obliquely straddled the edge of a mesoproterozoic continental margin as a result of this geometry the rift zone died out to the north as it propagated into cold archean crust elzevirian subduction was probably halted by the collision of the continental margin with an offshore arc and later with a major continental block giving rise respectively to the shawinigan and the ottawan orogenic events these collisional events can be grouped as parts of the grenville orogenic cycle

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

geological correlations between cape breton island and newfoundland northern appalachian orogen geological correlations between cape breton island and newfoundland are apparent both in surface geology and at deeper crustal levels based on similarities in sm nd isotopic signatures the mira terrane of southeastern cape breton island is part of the avalon terrane sensu stricto and is composed of neoproterozoic volcanic sedimentary plutonic belts and overlying cambrian rocks directly comparable to those in the western part of the newfoundland avalon terrane the bras d or terrane is also mainly of neoproterozoic age but shows lithological and isotopic contrasts with the mira terrane small areas of similar neoproterozoic rocks occur in southern newfoundland and to the north as inliers in the exploits terrane the bras d or terrane and similar rocks in newfoundland are interpreted to represent a peri gondwanan terrane where rocks of the gander terrane were later formed hence this area is part of the central mobile belt and distinct from avalon terrane sensu stricto the aspy terrane is a complex area

crystal size variations in eemian age ice from the grip ice core central greenland continuous measurements of ice crystal size have been carried out on an 80 m sequence between 2790 and 2870 m depth in the grip ice core from central greenland the ice in this interval is at present considered to originate from the eemian interglacial period the record reveals that the crystal size in ice older than 100 000 yr is highly dependent on climatic conditions at the time of snowfall this dependence shows up as a strong correlation between delta 18 o values and crystal size throughout the eemian as well as a negative correlation between crystal size and several soluble and insoluble impurities although high resolution impurity records are available from selected parts of the eemian ice the study is not conclusive on which impurities are most effective in slowing grain growth it is shown that the normal grain growth process commonly observed in the upper few hundred metres of polar ice sheets does not yield grain sizes compatible with observed ones at this depth in the ice sheet even in those parts of the eemian ice where impurity drag effects are not present polygonization of crystals within the ice sheet and the nucleation and rapid growth of new grains at relatively high temperatures in the lowest part probably play an important role in producing the observed grain size variations the relevance of possible flow disturbances of the grip eemian climatic record for the results presented is discussed briefly

a hafnium isotopic record of magmatic arcs and continental growth in the iapetus ocean the contrasting evolution of ganderia and the peri laurentian margin abstract we test the sensitivity of combined u pb hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens using ganderia from the canadian appalachian orogen as an example ganderia hosted a long lived magmatic arc that began at least by 650 ma and continued until 450 ma during which time it drifted northward from the amazonian margin its isotopic record suggests arc activity began earlier possibly at 750 ma proto ganderia within an oceanic domain the u pb hf zircon isotopic array suggests that ganderia formed on a sliver of paleoproterozoic mesoproterozoic basement tdmc 2 2 1 1 ga a transition towards more evolved hf isotopic compositions between 650 and 600 ma is interpreted to coincide with its accretion to the amazonian margin of gondwana consistent with other geological evidence accretion was followed by increasing amounts of juvenile crustal inputs between 550 and 500 ma which is interpreted to reflect subduction roll back and the onset of drift from the gondwanan margin trench retreat culminated in the separation of the ganderia arc and formation of the exclusively oceanic 510 450 ma penobscot popelogan victoria arcs on ganderia accretion of ganderia to the active notre dame subzone along the laurentian margin occurred in the late ordovician and is marked by the abrupt termination of ganderian arc magmatism the notre dame arc evolved along the laurentian margin between 515 and 425 ma and was built on paleoproterozoic archean t dm c 2 9 2 5 ga crust supra subduction zone magmatism occurred at 490 460 ma during the closure of the taconic seaway and was dominated by the recycling of laurentian crust with progressive mixing of evolved and juvenile arc magmas over time the preservation of very evolved notre dame arc zircon in ganderian overstep sequences confirms the arrival of the leading edge of ganderia to laurentia by 450 ma the hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems

episodic mafic crust formation from 4 5 to 2 8 ga new evidence from detrital zircons slave craton canada the hf and 18o values in detrital zircons from the slave craton canada indicate three episodes of crust formation between ca 4 5 and 2 8 ga namely at ca 4 4 4 5 ga ca 3 8 ga and ca 3 4 ga most of the juvenile crust appears to have been mafic in composition and there is no clear evidence for initial granitic protocrust in the hadean of the slave craton the range of initial hf values in zircons increases from 3 9 to 2 8 ga indicating that both extraction of new material from mantle and reworking of the older crust are important for the secular evolution of the continental crust a preliminary review of available hf data in zircons suggests that the three episodes of crust generation may have been of global importance the mafic crust formed in the archean and the hadean was then reworked for at least 0 5 1 5 b y as indicated by data from the slave craton gondwana and the limpopo belt of africa

orbit related long term climate cycles revealed in a 12 myr continental record from lake baikal quaternary records of climate change from terrestrial sources such as lake sediments1 2 and aeolian sediments3 4 in general agree well with marine records5 6 but continuous records that cover more than the past one million years were essentially unavailable until recently when the high sedimentation rate site of lake baikal was exploited1 2 7 8 because of its location in the middle latitudes lake baikal is highly sensitive to insolation changes9 and the entire lake remained uncovered by ice sheets throughout the pleistocene epoch making it a valuable archive for past climate here we examine long sediment cores from lake baikal that cover the past 12 million years our record reveals a gradual cooling of the asian continental interior with some fluctuations spectral analyses reveal periods of about 400 kyr 600 kyr and 1 000 kyr which may correspond to milankovitch periods reflecting orbital cycles our results indicate that changes in insolation were closely related to long term environmental variations in the deep continental interior over the past 12 million years

sm nd isotopic geochemistry of precambrian to paleozoic granitoid suites and the deep crustal structure of the southeast margin of the newfoundland appalachians in the eastern central mobile belt of the newfoundland appalachians late precambrian basement inliers have end from 3 to 2 but cambro ordovician metasedimentary rocks have initial end below 7

a 200 ka geomagnetic chronostratigraphy for the labrador sea indirect correlation of the sediment record to specmap abstract a chronostratigraphic framework for the last 200 ka based on relative variations of geomagnetic paleointensity has been derived for the labrador sea where 18o stratigraphies are compromised by lack of benthic foraminifera and environmental perturbations of the planktic record relative paleointensity records derived from three labrador sea piston cores are correlated to a recently developed 200 ka paleointensity composite sint 200 y guyodo j p valet earth planet sci lett 143 1996 23 36 the chronology for sint 200 was derived by correlating oxygen isotope stratigraphies from individual paleointensity records to the specmap chronostratigraphy the correlation of sint 200 to labrador sea paleointensity records allows the specmap chronology to be imported into the labrador sea although comparisons of the chronostratigraphy based on paleointensity correlations with those derived from ams 14c dates and 18o data indicate significant discrepancies support for the new chronostratigraphic framework is provided by 1 consistent correlation of labrador sea detrital layers to north atlantic heinrich layers 2 general consistency between planktic 18o values and the specmap reference curve and 3 feasible prediction for the age of ash layer 2 as the new chronostratigraphy is tied to specmap labrador sea sediment cores can now be directly compared with any specmap dated record

birth and demise of the rheic ocean magmatic arc s combined u pb and hf isotope analyses in detrital zircon from sw iberia siliciclastic strata paleozoic continental reconstructions indicate that subduction of rheic oceanic lithosphere led to collision between laurussia and gondwana which was a major event in the formation of the ouachita appalachian variscan orogenic belt and the amalgamation of pangea however arc systems which record rheic ocean subduction are poorly preserved the preservation of devonian detrital zircon in late devonian early carboniferous siliciclastic rocks of sw iberia rather than arc related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion here we report in situ detrital zircon u pb isotopic analyses of late devonian early carboniferous siliciclastic rocks from the pulo do lobo zone which is a reworked late paleozoic suture zone located between laurussia and gondwana detrital zircon age spectra from the pulo do lobo zone frasnian formations show striking similarities revealing a wide range of ages dominated by neoproterozoic and paleoproterozoic grains sourced from rocks typical of peri gondwanan terranes such as avalonia the meguma terrane and the ossa morena zone pulo do lobo rocks also include representative populations of mesoproterozoic and early silurian zircons that are typical of avalonia and the meguma terrane which are absent in the ossa morena zone the famennian tournaisian formations from the pulo do lobo zone however contain more abundant middle late devonian zircon indicating the contribution from a previously unrecognized source probably related to the rheic ocean magmatic arc s the middle late devonian to early carboniferous zircon ages from the siliciclastic rocks of sw iberia south portuguese pulo do lobo and ossa morena zones have a wide range in ehft values 8 2 to 8 3 indicating the likely crystallization from magmas formed in a convergent setting the missing rheic ocean arc was probably built on a meguma avalonia type basement we propose for the pulo do lobo zone that the frasnian sedimentation occurred through the opening of a back arc basin formed along the laurussian active margin during rheic ocean subduction as has been recently proposed for the rhenohercynian zone in central europe detrital zircon ages in the frasnian siliciclastic rocks indicate provenance in the meguma terrane avalonia and devonian rheic ocean arc s as a result of back arc basin inversion the frasnian formations underwent deformation metamorphism and denudation and were unconformably overlain by famennian to visean siliciclastic strata including the phyllite quartzite formation of the south portuguese zone the latest devonian early carboniferous detritus were probably shed to the pulo do lobo zone represa and santa iria formations by recycling of devonian siliciclastic rocks from the south portuguese zone meguma terrane and from a new distinct source with baltica laurentia derivation preserved in the horta da torre formation and alajar melange

long term secular variation of the geomagnetic field during the last 200 kyr recorded in sediment cores from the western equatorial pacific we have conducted a paleomagnetic study of five hemipelagic clay cores of 4 7 m in length obtained from the west caroline basin western equatorial pacific the purpose of this study is to clarify the long term secular variation of the geomagnetic field during the last 200 kyr using both intensity and directional changes one core was dated using oxygen isotope ratios 18o and characteristic magnetic susceptibility variations of about a factor of four allowed precise correlation between all cores the magnetic mineralogy is estimated to be dominated by magnetite of single domain sd and or pseudo single domain psd state the magnetic mineralogy and grain size are uniform throughout the cores which enabled us to obtain relative paleointensity variations as well as directional changes remanent intensity normalized by anhysteretic remanent magnetization arm indicates that sharp intensity drops to 20 of the present field occurred at about 40 and 190 kyr b p with a broader decrease around 110 kyr b p the former two are accompanied by anomalous remanent directions suggesting short polarity reversals or excursions their ages are close to those of the previously reported laschamp excursion and biwa i event a prominent intensity peak occurred immediately preceding the possible excursion at 40 kyr b p our results are in general similar to the recently reported datasets spanning the last 80 140 kyr from the somali basin the mediterranean and the sulu sea 1 3 the inclination record shows long term cyclic changes of several degrees in amplitude the intervals of the recurring variation are 40 50 kyr which is longer than the core s memory and is close to the milankovitch frequency associated with the change in obliquity of earth s rotational axis we infer that an external force such as orbital forcing could be a cause of the long term secular variation

tectono stratigraphic terranes in cape breton island nova scotia implications for the configuration of the northern appalachian orogen cape breton island is divided into four terranes on the basis of contrasts in pre carboniferous geology the blair river complex in the north is an exposure of north american grenvillian basement analogous to the humber zone basement in newfoundland ordovician to devonian metavolcanic metasedimentary gneissic and granitic rocks of the aspy terrane are correlative with parts of the gander terrane of newfoundland and new brunswick the bras d9or terrane characterized by low pressure gneisses a carbonate clastic platform sequence and late precambrian early cambrian plutons may be correlative with units previously included in the gander terrane in southern newfoundland and the avalon terrane in southern new brunswick the mira terrane in southeastern cape breton island including late precambrian to early cambrian volcanic and sedimentary sequences and fossiliferous cambrian ordovician units is clearly part of the avalon terrane therefore with the exception of the dunnage terrane which is not represented in cape breton island the terranes of newfoundland continue through cape breton island they are offset to the northwest to the mainland part of the appalachian orogen in new brunswick

mantle and crustal sources of archean anorthosite a combined in situ isotopic study of pb pb in plagioclase and lu hf in zircon isotopic analyses of ancient mantle derived magmatic rocks are used to trace the geochemical evolution of the earth s mantle but it is often difficult to determine their primary initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration we present in situ analyses by la mc icpms for the pb isotopic compositions of igneous plagioclase an75 89 megacrysts and the hf isotopic compositions of bse imaged domains of zircon grains from two mantle derived anorthosite complexes from south west greenland fiskenaesset and nunataarsuk which represent two of the best preserved archean anorthosites in the world in situ la icpms u pb geochronology of the zircon grains suggests that the minimum crystallization age of the fiskenaesset complex is 2 936 13 ma 2 mswd 1 5 and the nunataarsuk complex is 2 914 6 9 ma 2 mswd 2 0 initial hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ehf units in terms of pb isotopic compositions of plagioclase both anorthosite complexes share a depleted mantle end member yet their pb isotopic compositions diverge in opposite directions from this point fiskenaesset toward a high more radiogenic pb crustal composition and nunataarsuk toward low less radiogenic pb crustal composition by using hf isotopes in zircon in conjunction with pb isotopes in plagioclase we are able to constrain both the timing of mantle extraction of the crustal end member and its composition at fiskenaesset the depleted mantle melt interacted with an eoarchean 3 700 ma mafic crust with a maximum 176lu 177hf 0 028 at nunataarsuk the depleted mantle melt interacted with a hadean 4 200 ma mafic crust with a maximum 176lu 177hf 0 0315 evidence from both anorthosite complexes provides support for the long term survival of ancient mafic crusts that although unidentified at the surface to date could still be present within the fiskenaesset and nunataarsuk regions

a paleomagnetic power spectrum we construct a power spectrum of geomagnetic dipole moment variations or their proxies that spans the period range from some tens of million down to about 100 years empirical estimates of the spectrum are derived from the magnetostratigraphic time scale from marine sediment relative paleointensity records and from a time varying paleomagnetic field model for the past 7 kyr the spectrum has the most power at long periods reflecting the influence of geomagnetic reversals and in general decreases with increasing frequency decreasing period the empirical spectrum is compared with predictions from simple models discrepancies between the observed and predicted spectra are discussed in the context of i changes in reversal rate ii overall average reversal rate iii cryptochrons iv the time taken for a reversal to occur and v long term paleosecular variations and average estimates of the field strength and variance from other sources 2005 elsevier b v all rights reserved

limitations on stratigraphic analyses due to incomplete age control and their relevance to sedimentary paleomagnetism abstract a major limitation in the analysis of physical quantities measured from a stratigraphic core is incomplete knowledge of the depth to age relationship for the core records derived from diverse locations are often compared or combined to construct records that represent a global signal time series analysis of individual or combined records is commonly employed to seek quasi periodic components or characterize the timescales of relevant physical processes assumptions that are frequently made in the approximation of depth to age relationships can have a dramatic and harmful effect on the spectral content of records from stratigraphic cores a common procedure for estimating ages in a set of samples from a stratigraphic core is to assign based on complementary data the ages at a number of depths tie points and then assume a uniform accumulation rate between the tie points imprecisely dated or misidentified tie points and naturally varying accumulation rates give rise to discrepancies between the inferred and the actual ages of a sample we develop a statistical model for age uncertainties in stratigraphic cores that treats the true but in practice unknown ages of core samples as random variables for inaccuracies in the ages of tie points we draw the error from a zero mean normal distribution for a variable accumulation rate we require the actual ages of a sequence of samples to be monotonically increasing and the age errors to have the form of a brownian bridge that is the errors are zero at the tie points the actual ages are modeled by integrating a piecewise constant randomly varying accumulation rate in each case our analysis yields closed form expressions for the expected value and variance of resulting errors in age at any depth in the core by monte carlo simulation with plausible parameters we find that age errors across a paleomagnetic record due to misdated tie points are likely of the same order as the tie point discrepancies those due to accumulation rate variations can be as large as 30 kyr but are probably less than 10 kyr we provide a method by which error estimates like these can be made for similar stratigraphic dating problems and apply our statistical model to an idealized marine sedimentary paleomagnetic record both types of errors severely degrade the spectral content of the inferred record we quantify these effects using realistic tie point ages their uncertainties and depositional parameters

tectonic significance of the late proterozoic economy river gneiss cobequid highlands avalon composite terrane nova scotia a 734 2 ma u pb zircon age for the economy river orthogneiss coboquid highlands nova scotia is interpreted as being representative of a regionally extensive ca 820 660 ma event that is recorded in many parts of the late proterozoic early cambrian avalon composite terrane and the gondwanan margin the geochemistry of the gneiss is consistent with an arc environment although the gneiss may represent part of the sialic basement to the terrane field relationships indicate that some of the basement is significantly older the date may provide a minimum age for the platformal sedimentary rocks gamble brook formation that the orthogneiss intruded and thus help constrain the late proterozoic paleogeographic position of avalon relative to gondwanaland

an autochthonous avalonian basement source for the latest ordovician brenton pluton in the meguma terrane of nova scotia u pb hf isotopic constraints and paleogeographic implications the ediacaran ordovician meguma supergroup was thrust over avalonia basement prior to the intrusion of post acadian ca 370 ma s type granitic batholiths this has led to two main hypotheses regarding the original location of the meguma terrane a continental rise prism bordering either nw africa or avalonia on the other hand the pre acadian ca 440 ma brenton pluton has yielded the following u pb la icp ms zircon data 1 448 3 ma population peak inferred to be the intrusive age and 2 ca 550 and 700 ma inherited ages common to both avalonia and nw africa in contrast hf isotopic analyses of zircon yielded model ages ranging from 814 to 1127 ma with most between 940 and 1040 ma such ages are typical of avalonia and not nw africa the ages of the inherited zircons found within the brenton pluton suggest that it was probably derived by partial melting of sub meguma mid crustal avalonian rocks upon which the meguma supergroup was deposited although avalonia is commonly included in the peri gondwanan terranes off nw africa or amazonia paleomagnetic data faunal provinciality and hf data suggest that during the ediacaran early cambrian it was an island chain lying near the tropics ca 20 30 s and was possibly a continuation of the bolshezemel volcanic arc accreted to northern baltica during the ediacaran timanide orogenesis this is consistent with the similar derital zircon population in the ediacaran cambrian meguma supergroup and the dividal group in northeastern baltica

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

550 580 ma magmatism in cape breton island nova scotia canada the product of nw dipping subduction during the final stage of amalgamation of gondwana abstract cape breton island is host to widespread magmatic activity dated between 550 and 580 ma in the south its products include the capelin cove pluton and host coastal volcanic belt both are composed of bimodal tholeiitic calc alkaline suites that were emplaced on a thin sialic crust in a volcanic arc system their trace element and nd isotope variations are consistent with fractional crystallization accompanied by lower crustal contamination in a compositionally zoned magma chamber in contrast the indian brook and kerrs brook plutons and price point volcanic rocks in central cape breton island have higher contents of strongly incompatible trace elements and resemble modern arc related calc alkaline rocks emplaced on a moderately thick continental crust comparison with available data for other 550 580 ma igneous rocks across cape breton island indicates that they were produced above a nw dipping subduction zone this convergence was synchronous with the final stage of globalscale orogenic activity that culminated with the final amalgamation of gondwana available detrital zircon ages in late neoproterozoic 650 545 ma sediments in the antigonish highlands avalon composite terrane suggest derivation from the amazon craton it is proposed that the avalon composite terrane formed off northwestern south america during the late neoproterozoic

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

normalized remanence in sediments of the western equatorial pacific relative paleointensity of the geomagnetic field piston cores from the ontong java plateau collected during the eurydice expedition have been used to estimate regional geomagnetic paleointensity rock magnetic measurements of the sediments show that 1 the remanence is carried by stable pseudo single domain magnetite 2 the grain sizes and concentrations are relatively uniform with respect to the normalization technique and 3 susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field we therefore have measured the remanent intensity both natural and demagnetized to 15 mt and susceptibility of archive quarters of three cores at centimeter intervals using an automated through bore magnetometer a further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores one of our cores displays high coherence over a broad range of frequencies while for two others coherence is insignificantly different from zero at the 95 level of confidence oxygen isotopic records paleontological datum levels and the brunhes matuyama boundary provide an independent temporal framework for inter core comparison when the two cores which pass our acceptance criteria are placed on a common time scale the coherence between the normalized intensity records is significantly different from zero at well above the 95 level of confidence suggesting some common input signal over the same band of frequencies furthermore there is a great deal of similarity between the records from the ontong java plateau and a record from core rc10 167 taken some 3000 km to the north the distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows 0 5 ma with a slightly smaller standard deviation the inter core agreement within a single region the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years over the period 0 350 ka this feature is however lacking for the period 350 700 ka

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

petrogenesis of siluro devonian rhyolites of the tobique group in the northwestern appalachians northern new brunswick canada tectonic implications for the accretion history of peri gondwanan terranes along the laurentian margin

calibration of cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic atlantic ocean a stratigraphic tool for carbonate poor quaternary sediments we calibrated the cycladophora davisiana abundances versus oxygen isotope stratigraphy back to220ka for the subantarctic atlantic ocean the relative abundances of c davisiana and 18omeasurements of benthic and planktic foraminifera have been determined in two sediment cores oxygen isotope stratigraphy has been used to date the c davisiana records and to assignspecmap ages to the c davisiana events comparisons with an existing calibration from thesubantarctic indian ocean show that the c davisiana events b2 c1 c2 d e1 e2 e3 f h i1 and i2occur synchronous within the errors of the oxygen isotope stratigraphy in the indian and the atlanticsectors of the southern ocean larger deviations occur only for events b1 and g furthermore the long term fluctuations in c davisiana abundances have been studied in a sediment core coveringthe last 700kyr based on biostratigraphic extinction levels ages for early brunhes c davisianaevents have been estimated major c davisiana abundance maxima occur approximately every100ka in conjunction with glacial interglacial cycles over the entire record

contrasting u pb ages from plutons in the bras d or and mira terranes of cape breton island nova scotia u pb dates from zircon titanite and monazite in plutons of the bras d or and mira terranes of southern cape breton island combined with 40ar 39ar and other radiometric age data indicate that th

electrical measurements on the greenland ice sheet project 2 core the greenland ice sheet project 2 gisp2 electrical conductivity measurement ecm record is an indication of the concentration of h in the core the ecm detected seasonal variations in the nitrate concentration of the core which were used to assist in dating the core by annual layer counting volcanic eruptions that produce acidic aerosols are recorded in the ecm record evidence of biomass burning is detected by the ecm because fire related ammonium emissions neutralize the acids in the core rapid climate transitions associated with the younger dryas and dansgaard oeschger interstadial events alter the concentration of alkaline dust and are detected by the ecm the ecm has been used to develop stratigraphic ties between the gisp2 and the greenland ice core project cores users of the data should be aware of some instrument related artifacts in the ecm record

late neoproterozoic arc related magmatism in the horse cove complex eastern avalon zone newfoundland the horse cove complex hcc is a recently recognized late ediacaran swarm of mafic to felsic dykes that occurs in the newfoundland avalon zone it is focused along the extrapolated trace of the to

geomagnetic modulation of the 36cl flux in the grip ice core greenland geomagnetic field strength is expected to affect the production rate of cosmogenic isotopes such as beryllium 10 carbon 14 or chlorine 36 chlorine 36 data from the greenland ice core project grip ice core agree well with a production rate calculation based on a paleomagnetic reconstruction for the past 100 000 years over both long and short term variations a chlorine 36 peak at 38 000 years ago previously found in the beryllium 10 record from the vostok ice core can be explained by a period of low geomagnetic field intensity

nd isotopic compositions of felsic igneous rocks in cape breton island nova scotia nd isotopic data from 18 felsic plutonic and volcanic units in cape breton island show variations consistent with other geological and geophysical evidence for at least three distinct terranes a ca 1 2 ga syenite considered to be part of grenvilleage basement exposed in the northwestern part of the island yields an initial end value of 0 4 and a depleted mantle model age tdm of 1 66 ga suggesting substantial involvement of older archean or early proterozoic crust in its petrogenesis a tdm of 1 38 ga indicated by nd isotopic data for a devonian granite spatially associated with the syenite is also consistent with the presence of older crust in that part of cape breton island in contrast silurian rhyolite and devonian granites from the aspy terrane have tdm ranging from 0 7 to 1 1 ga and initial end between 2 8 and 1 2 and overlap in isotopic characteristics with late precambrian and early ordovician plutons in the bras d or terrane that yield tdm of 0 9 1 1 ga and initial end of 1 4 to 1 8

the early to middle pleistocene boundary in the baza basin spain abstract the continental vertebrate fauna of cullar baza 1 granada spain occur immediately above the matuyama brunhes polarity boundary and therefore represent an initial record for the middle pleistocene all polarity zones for the late pliocene through middle pleistocene are found in an 80 m section which includes this fossil quarry the existing collection of abundant remains of micro mammals macro mammals and lithic artifacts indicating human activity can now be assigned an earliest middle pleistocene age of 0 75 ma another section 25 km across the neogene baza basin also has the matuyama brunhes polarity boundary however in this case the huescar 1 fossil quarry and the puerto lobo sites are both below the polarity boundary thus representing a record of the late early pleistocene at 0 9 ma differences in micro mammal species across these matuyama brunhes boundaries are significant and justify an approximately coincident biostratigraphic boundary between the biharian and toringian stages

ams radiocarbon dating of annually laminated sediments from lake holzmaar germany ams radiocarbon ages have been determined on terrestrial macrofossils selected from the annually laminated sediments of lake holzmaar germany the radiocarbon chronology of this lake covers the last 12 6 ka comparison of the radiocarbon dated varve chronology with tree ring data shows that an additional 878 years have to be added to the varve chronology the corrected c 14 varve chronology of holzmaar reaches back to ca 13 8 ka cal bp and compares favourably with the results from soppensee switzerland hajdas et al 1993 the corrected ages for the onset and the end of the younger dryas biozone are 11 940 cal bp and 11 490 cal bp respectively the ash layer of the laacher see volcanic eruption is dated at 12 201 224 cal bp and the ulmener tephra layer is dated at 10 904 cal bp

magnetostratigraphy of the orce region baza basin se spain new chronologies for early pleistocene faunas and hominid occupation sites abstract the orce fossil quarries in the baza basin of southeastern spain are a rich source of early pleistocene palaeolithic tools and vertebrate remains geologic fieldwork during the last decade has placed these fossiliferous strata within the context of a thick neogene continental sequence detailed lithostratigraphic and magnetostratigraphic results indicate that at least the upper 60 m of this sequence are of early pleistocene age the quarried strata venta micena barranco leon and fuentenueva 3 are from a narrow time span gazella borbonica equus stenonis and the beginning of more characteristic pleistocene fauna in the venta micena site with hippopotamus antiquus equus granatensis homo sp thus far no evidence for human occupation has been found within the earlier fuentenueva 1 quarry although many of the same terminal mn17 species have been found with hominids on the other side of europe at the dmanisi site earliest pleistocene in the republic of georgia

the effects of aliasing and lock in processes on palaeosecular variation records from sediments studies of sedimentary records of palaeointensity variation report periods as long as 50 kyr archaeointensity data show geomagnetic periods of 2 kyr with large ampli tudes sampling of the sedimentary records can be as coarse as 8 kyr so the apparent long periods could be caused by aliasing the sedimentary lock in process could smooth the record and remove short periods thereby preventing aliasing from occurring we examine possible effects of aliasing by creating a 100 kyr long synthetic sequence of palaeointensity variation with a similar spectrum to that of archaeomagnetic data from the last 12 kyr and resampling at longer intervals with no lock in smoothing aliasing produces spurious energy in the spectra at long periods when smoothing by the sedimentation process is applied the amplitudes of the aliased peaks are reduced but still cause significant spurious long period energy in the spectra for some sedi mentation rates we restrict our analysis to palaeointensity data but similar problems may also exist for coarsely sampled directional data to avoid aliasing we recommend a maximum sampling interval of 2 kyr

altering avalonia oxygen isotopes and terrane distinction in the appalachian peri gondwanan realmlaboratory for stable isotope science lsis the university of western ontario contribution 236 distinct 18o depletion is characteristic of a large majority of the 620 550 ma felsic igneous rocks of avalonia in the northern appalachian orogen neoproterozoic rocks in the boston avalon terrane have the lowest 18owr values 3 1 followed by the mira terrane in cape breton island and the caledonia terrane in new brunswick 1 2 the avalon terrane in newfoundland 2 8 and the antigonish highlands in nova scotia 5 3 in contrast this depletion of 18o is observed in very few of the paleozoic felsic igneous rocks from these avalonian terranes and also in very few of the neoproterozoic and paleozoic felsic igneous rocks from the inboard ganderian terranes the low 18o character of the neoproterozoic igneous rocks is related to regional pervasive post magmatic alteration by predominantly meteoric hydrothermal fluids 18oh2o 6 to 4 at 200 450 c the alteration likely occurred during late neoproterozoic transtensional extension of avalonia large scale fluid infiltration and circu

a depth derived pleistocene age model uncertainty estimates sedimentation variability and nonlinear climate change 1 a new chronology of glaciation spanning the last 780 000 years is estimated from 21 marine sediment cores using depth as a proxy for time to avoid biasing this depth derived age estimate the depth scale is first corrected for the effects of sediment compaction to provide age uncertainty estimates the spatial and temporal variability of marine sediment accumulation rates are estimated and modeled as an autocorrelated stochastic process depth derived ages are estimated to be accurate to within 9000 years and within this uncertainty are consistent with the orbitally tuned age estimates nonetheless the remaining differences between the depth and orbitally tuned chronologies produce important differences in the spectral domain from the 18o record using the depth derived ages we infer that there are weak nonlinearities involving the 100 kyr and obliquity frequency bands which generate interaction bands at sum and difference frequencies if an orbitally tuned age model is instead applied these interactions are suppressed with the system appearing more nearly linear

u pb geochronologic constraints on the volcanic evolution of the mira avalon terrane southeastern cape breton island nova scotia new u pb ages for late precambrian volcanic and associated plutonic units in the mira avalon terrane of southeastern cape breton island indicate that volcanic suites were erupted over a span of a

relative variations in geomagnetic intensity from sedimentary records the past 200 000 years abstract eighteen records of relative paleointensity from marine sediments have been compiled in order to retrace the global changes in geomagnetic intensity during the past 200 kyr before dealing with any other source of noise we focused on problems inherent to the depth versus time correlations the accuracy and resolution of dating mostly deduced from isotopic stratigraphy were identified as significant factors of distortion we found also that different time scales have been used by authors after readjustment with respect to the same time scale the records were stacked together and the statistical significance of the results tested by a bootstrap technique seventeen records shown to be coherent were combined into a synthetic curve defined as sint 200 other stacks combining various subsets of data were found to be identical to sint 200 except for changes in dispersion in fact the dispersion around sint 200 could be reduced further by improving the correlations without violating the stratigraphy a procedure which at least for some records emphasizes the need for more detailed depth time correlations another characteristic emerging from the results is that paleointensity features as short as 10 kyr are observed between 120 200 ka and can thus in principle be resolved however subsequent refinement is still necessary for this period which is documented by fewer records than the following 120 kyr finally we compared sint 200 with the records from the labrador sea and the lac du bouchet ldb both of which rely mostly on 14c chronology the two records show satisfactory agreement with sint 200 when transferred with respect to the age model established by correlating their 18o labrador and susceptibility features ldb with the 18o reference record the offset of 7 kyr at 40 ka measured in both cases between the 14c and 18o ages is higher than the value deduced from a recent model of a geomagnetic field control of 14c production 1 but is not inconsistent with recent u th dates

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

nd isotopic characteristics of terranes within the avalon zone southern new brunswick nd isotopic data are presented for rock units in four terranes within the traditional avalon zone of southern new brunswick initial end values for igneous rocks within the caledonia terrane range from 1 6 to 4 6 whereas values for sedimentary rocks range from 8 4 to 3 6 a granite within the kingston terrane has end 438 ma 4 0 nd isotopic compositions for the kingston and caledonia terranes are similar to those of the mira terrane in cape breton island the antigonish highlands of nova scotia and plutonic rocks of eastern newfoundland each of these regions may be a dismembered part of a single terrane the avalon terrane sensu stricto initial end values for rocks from the brookville terrane range from 1 3 to 2 8 the coverdale anorthosite within the brookville terrane has a present day end value of 12 1 and a depleted mantle model age of 1 3 ga similar to mesoproterozoic anorthosites in laurentia clastic sedimentary rocks in the green head group have end 750 ma values of 2 0 and 10 9

a high resolution late quaternary upwelling record from the anoxic cariaco basin venezuela results are presented of a high resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the cariaco basin in the southern caribbean sea the cariaco basin is a small anoxic marine basin on the northern continental margin of venezuela in an area today characterized by both seasonal trade wind induced upwelling and pronounced dry and wet seasons our data indicate that large changes in the intensity of upwelling and hence trade wind strength occurred in this region during the last glacial interglacial transition and throughout the holocene during the last glacial lowstand of sea level the cariaco basin was effectively isolated from the open caribbean along its northern margin by the then largely emergent tortuga bank oxic conditions existed in the deep cariaco basin at this time and surface productivity was low about 12 600 years ago the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the laurentide ice sheet into the gulf of mexico strong upwelling between 12 600 and about 10 000 years ago may be related to intensified trade winds resulting in part from cooler sea surface temperatures in the caribbean and gulf of mexico after about 10 000 years ago upwelling intensity was reduced though highly variable a preliminary frequency domain analysis of the holocene portion of the cariaco basin time series suggests that solar forcing may explain a significant component of the century scale variability observed in the record of upwelling and trade wind strength

reappraising the neoproterozoic east avalonian terranes of southern great britain abstract inliers of late neoproterozoic basement rocks exposed in southern great britain described in summary herein have been previously attributed to a series of terranes bounded by major crustal lineaments that form a dissected remnant of a single subduction complex together these terranes have been termed east avalonia and interpreted as correlative with west avalonia of the northern appalachian orogen a review of age and nd and o isotopic data shows an essentially ensialic complex with two broad overlapping divisions 1 the monian composite terrane of nw wales and fenland terrane of eastern england that are characterised by felsic intrusions at around 615 ma and have more evolved end t 7 5 to 2 5 older t dm 1868 ma to 1269 ma and generally more elevated 18 o up to 13 7 and 2 a more diverse assemblage represented by the cymru wrekin and charnwood terranes including more substantive volcano sedimentary successions and intrusions ranging in age between around 710 ma and 566 ma in which 615 ma magmatic rocks are not represented less evolved end t 2 5 to 3 5 younger t dm 1501 994 ma and lower 18 o 6 5 to 10 1 comparison with other neoproterozoic basement units throughout the northern appalachian orogen shows that both divisions from southern great britain have closer isotopic affinity with igneous components of ganderia which generally have evolved compositions interpreted to reflect an ensialic setting and the input of gondwanan cratonic source components they are mostly dissimilar from magmatic rocks of west avalonia which typically have less evolved compositions interpreted to reflect the contribution of juvenile mesoproterozoic source components

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr