comparative stratigraphy and subsidence history of mesozoic rift basins of north atlantic mesozoic rift basins around the north atlantic all possess many of the following upper jurassic to lower cretaceous oxfordian to albian stratigraphic elements 1 an early oxfordian hiatus 2 backfill deposits in upper jurassic paleovalleys 3 multiple tithonian transgressive regressive sequences 4 early berriasian regressive sequences 5 a sub valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates 6 valanginian to hauterivian fining upward sequences 7 condensed barremian sequences and 8 prominent aptian unconformities more local features include abrupt kimmeridgian deepening lusitanian basin portugal albian deepening cantabrian and aquitaine basins northern spain and southern france and late albian early cenom nian source rejuvenation cantabrian basin northern iberia decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence relatively high rates 50 100 m m y characterize onset of rift phases in the triassic and the kimmeridgian locally in the lusitanian basin of portugal kimmeridgian rates reach about 250 m m y in the cantabrian and aquitaine basins a third phase of rapid subsidence characterizes the aptian albian interval after the main phase of continental breakup around iberia this phase may be due to transtensional stresses as iberia continued to rotate relative to europe tensional intraplate stresses or gravity driven partial collapse of the new continental marg n the north atlantic mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of europe and north america pulses of extension generally are reflected in the stratigraphy of all of the basins superimposed on this record are the effects of eustatic sea level variations the main difference between the canadian jeanne d arc basin and iberian basins is the significantly greater early and middle jurassic basement subsidence rate in the former about 40 m m y vs 0 10 m m y this difference may result from prolonged extension on the canadian side perhaps due to northward transfer of shear stress from left lateral transform motion between the grand banks and northwestern africa

bioturbation influence on reservoir quality a case study from the cretaceous ben nevis formation jeanne d arc basin offshore newfoundland canada the delineation ben nevis l 55 well located in the hebron ben nevis field offshore newfoundland targeted the cretaceous ben nevis formation in the petroleum rich jeanne d arc basin this case study focuses on the bioturbated net pay horizons and assesses the importance of animal sediment interactions in controlling the porosity and permeability of sandstone reservoir intervals our data reveal that bioturbation can either reduce permeability and porosity by as much as approximately 33 or enhance it by up to 600 dependent on burrow type and behavior of the trace making organism the net pay interval in the cored interval of ben nevis l 55 is characterized by ophiomorpha dominated ichnofabrics the action of bioturbators can be classified in terms of sediment mixing sediment cleaning sediment packing and pipe work building strategies bioturbation has the potential to 1 increase isotropy or uniformity of grain size by destroying sedimentary laminae through burrow homogenization or 2 decrease isotropy by selectively sorting grains into burrow lining and fill by grain size and through creation of open burrow systems filled with later sediments of differing character to the host sediment the petrophysical characteristics of the reservoir facies are highly dependent on trace fossil morphology presence or absence of burrow linings nature of burrow fills burrow size and bioturbation intensity mudstone rich facies and ichnofabrics containing mudstone filled and or lined burrows e g ophiomorpha and clusters of chondrites have the net effect of permeability reduction in contrast permeability enhancement is documented from muddy sandstone facies with clean sand filled burrows e g thalassinoides and clean sandstones with burrow mottled or diffuse to massive textures

climate volcanism feedback and the toba eruption of 74 000 years ago abstract a general feedback between volcanism and climate at times of transition in the quaternary climate record is suggested exemplified by events accompanying the toba eruption 74 000 yr ago the largest known late quaternary explosive volcanic eruption the toba paroxysm occurred during the 18o stage 5a 4 transition a period of rapid ice growth and falling global sea level which may have been a factor in creating stresses that triggered the volcanic event toba is estimated to have produced between 1015 and 1016 g of fine ash and sulfur gases lofted in co ignimbrite ash clouds to heights of at least 32 5 km which may have led to dense stratospheric dust and sulfuric acid aerosol clouds these conditions could have created a brief dramatic cooling or volcanic winter followed by estimated annual northern hemisphere surface temperature decreases of 3 to 5 c caused by the longer lived aerosols summer temperature decreases of 10 c at high northern latitudes adjacent to regions already covered by snow and ice might have increased snow cover and sea ice extent accelerating the global cooling already in progress evidence for such climate volcanic feedback following milankovitch periodicities is found at several climatic transitions

bristlecone pine tree rings and volcanic eruptions over the last 5000 yr many years of low growth identified in a western usa regional chronology of upper forest border bristlecone pine pinus longaeva and pinus aristata over the last 5000 yr coincide with known large explosive volcanic eruptions and or ice core signals of past eruptions over the last millennium the agreement between the tree ring data and volcano ice core data is high years of ring width minima can be matched with known volcanic eruptions or ice core volcanic signals in 86 of cases in previous millennia while there is substantial concurrence the agreement decreases with increasing antiquity many of the bristlecone pine ring width minima occurred at the same time as ring width minima in high latitude trees from northwestern siberia and or northern finland over the past 4000 5000 yr suggesting climatically effective events of at least hemispheric scale in contrast with the ice core records the agreement between widely separated tree ring records does not decrease with increasing antiquity these data suggest specific intervals when the climate system was or was not particularly sensitive enough to volcanic forcing to affect the trees and they augment the ice core record in a number of ways by providing confirmation from an alternative proxy record for volcanic signals by suggesting alternative dates for eruptions and by adding to the list of years when volcanic events of global significance were likely including the mid 2nd millennium bc eruption of thera

the ocean continent boundary off the western continental margin of iberia i crustal structure at 40 30 n summary the western continental margin of the iberian peninsula has the characteristics of a rifted non volcanic margin with half graben and tilted fault blocks seen in several places the ocean continent boundary ocb is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed as at many similar margins worldwide it is particularly useful to locate the ocb off western iberia in order to constrain the pre rift fit of north america to iberia and by implication the shape of the proto bay of biscay the fit is only marginally constrained by sea floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the ocb and it is even possible that all older oceanic crust was created during the cretaceous constant polarity interval the best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure therefore in 1986 a series of seismic refraction lines was shot parallel to and normal to the continental margin these lines enabled us to bracket the location of the ocb a further constraint on the location was obtained by modelling an east west magnetic profile which included the enigmatic j anomaly this anomaly can be explained as either just pre anomaly mo or as part of the cretaceous constant polarity interval depending on whether spreading began about 127 or after 118 myr ago respectively the evidence favours the former explanation lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles this indicated a new fracture zone at 41 15 n which offsets the ocb a few key reflection profiles also suggest that the ocb can be identified by an abrupt landward step down in acoustic basement we conclude that the ocb in the eastern iberia abyssal plain lies between 12 10 and 12 30 w and has a trend just east of north this westerly location is consistent with recent estimates of the location of the ocb off the grand banks but brings into question the proposed location at about 11 w of the ocb in the tagus abyssal plain

age and significance of sequence boundaries on jurassic and early cretaceous rifted continental margins the age and significance of sequence boundaries on jurassic to early cretaceous rifted continental margins in three ocean basins have been documented the margins are the santos basin in the south atlantic the grand banks in the north atlantic and the beaufort sea in the arctic ocean large industry data bases were used for the interpretation of each area megasequence boundaries separate the major phases of basin evolution for example syn rift and post rift boundaries developed with an average periodicity of 49 m y sequence boundaries define the component parts of each megasequence and developed with a modal periodicity of 10 15 m y out of 27 total boundary ages most 16 are developed on just one margin only two possible age ranges overlap on all three margins ighty percent of the megasequence boundaries and 50 of the sequence boundaries show a direct causal connection with coeval faulting and or folding the rest of the boundaries appear as unstructured surfaces separating transgressive and or regressive sedimentary wedges and are interpreted to result from changes in the rate of basin subsidence sediment input and long term eustatic sea level these data do not support theories advocating synchronous worldwide boundary development resulting from periodic short term falls in global eustatic sea level only in like basins of the same age with identical subsidence and sediment input rates are boundaries likely to develop synchronously hence the concept of global synchroneity of sequence boundary development may well be an illusion creat d by the similarity in age of the majority of basins studied as a result of this study it seems wise discard the global approach to basin analysis

evidence of the eldgja iceland eruption in the gisp2 greenland ice core relationship to eruption processes and climatic conditions in the tenth century glaciochemical studies and the evaluation of tephra in the gisp2 ice core provide information on the characteristics and potential environmental and climatic effects of the mid to late ad 930s voluminous fissure eruption of eldgja iceland the similarity in the chemical composition of basaltic glass shards found in a section of core dated at ad 938 4 compared to proximal glass from the eldgja eruption verifies the presence of eldgja debris a dacitic glass present in the same layer probably originated from eldgja as well in which case eldgja was the primary contributor of sulphur rich aerosols to the atmosphere in the late ad 930s we cannot completely exclude the possibility that another explosive eruption in the ad 930s produced this dacitic glass estimated maximum stratospheric loading is 100 x 1012 g h 2so4 over a 3 6 year period following the eruption but loading could be as low as half of that value a search of historical and proxy records for the late ad 930s to early 940s fail to show a co

ice core paleovolcanic records from the st elias mountains yukon canada 1 we previously reported a record of regionally significant volcanic eruptions in the north pacific using an ice core from eclipse icefield st elias mountains yukon canada the acquisition of two new ice cores from eclipse icefield along with the previously available eclipse icefield and mount logan northwest col ice cores allows us to extend our record of north pacific volcanism to 550 years before present using a suite of four ice cores spanning an elevation range of 3 5 km comparison of volcanic sulfate flux records demonstrates that the results are highly reproducible especially for the largest eruptions such as katmai a d 1912 correlation of volcanic sulfate signals with historically documented eruptions indicates that at least one third of the eruptions recorded in st elias ice cores are from alaskan and kamchatkan volcanoes although there are several moderately large volcanic explosivity index vei 4 eruptions recorded in only one core from eclipse icefield the use of multiple cores provides signals in at least one core from all known vei 4 eruptions in alaska and kamchatka since a d 1829 tephrochronological evidence from the eclipse ice cores documents eruptions in alaska westdahl redoubt trident and katmai kamchatka avachinsky kliuchevoskoi and ksudach and iceland hekla several unidentified tephra bearing horizons with available geochemical evidence suggesting alaskan and kamchatkan sources were also found we present a reconstruction of annual volcanic sulfate loading for the north pacific troposphere based on our ice core data and we provide a detailed assessment of the atmospheric and climatic effects of the katmai eruption

marine geology of sohm basin canadian atlantic margin multichannel seismic reflection seismic refraction and magnetic anomaly data are used to interpret the mesozoic sea floor spreading history and subsequent sedimentary evolution of the sohm basin southeast of nova scotia observed m series mesozoic magnetic anomalies show that oceanic crust in the sohm basin was generated at very slow rates of sea floor spreading 5 15 mm yr half rates the slow spreading created very rough crustal topography but the crustal thickness and seismic velocity structure are typical of normal oceanic basement basement depths corrected for sediment loading fall below the predicted age depth curve in the anomaly m25 m4 interval these depth anomalies and an associated regional negative geoid anomaly suggest that the sohm basin is underlai by asthenosphere denser and possibly colder than normal correlation of seismic reflection profiles in the basin to regional seismic stratigraphy together with age constraints from reflection pinch outs on dated oceanic crust provides a chronology for interpreting the seismic sequences the jurassic to lower cretaceous sequences are similar in thickness and acoustic character to sequences observed along the united states atlantic margin suggesting that the new england seamount chain had little effect on sediment distribution patterns within the early western north atlantic basin upper cretaceous sediments in the sohm basin are thin probably because seaward transport of sediment was restricted by eustatic sea level rise and by a sedimentary ridge uplifted because of salt tectonics along the central and upper continental rise progradati nal sedimentary wedges and erosional unconformities characterize the tertiary and quaternary sedimentary record along the lower continental rise the prograding sequences generally correlate to sea level lowstands repeated episodes of erosion by abyssal currents are marked by seismic unconformities horizon au lower oligocene lm lower miocene mm middle miocene and l upper pliocene the deep currents also controlled local patterns of deposition and modified cross slope sediment transport and accumulation patterns thick supra horizon l sediments cover the sohm basin and reflect rapid sediment influx caused by glacio eustatic fluctuations in sea level during the quaternary

timing and climate forcing of volcanic eruptions for the past 2 500 years volcanic eruptions contribute to climate variability but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual to decadal temperature variability in the northern hemisphere during the past 2 500 years our results are based on new records of atmospheric aerosol loading developed from high resolution multi parameter measurements from an array of greenland and antarctic ice cores as well as distinctive age markers to constrain chronologies overall cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth century pandemics famines and socioeconomic disruptions in eurasia and mesoamerica while allowing multi millennium quantification of climate response to volcanic forcing

impurity sources of f cl no3 and so4 2 in greenland and antarctic precipitation five polar impurity sources are proposed anthropogenic no3 and so42 in recent greenland but not antarctic snow marine aerosols the dominant cl source volcanic eruptions which produce detectable but variable peaks in so42 f and or cl concentrations biogenic sulfurous gases from soil and coastal waters the dominant source for so42 in nonvolcanic strata an unidentified source of background nitrate which is more active in the northern hemisphere and which displays a summer concentration maximum solar activity and supernova events do not modulate no3 concentrations in greenland snow

potential salinity driven free convection in a shale rich sedimentary basin example from the gulf of mexico basin in south texas we investigate the plausibility of salinity driven free thermohaline convection in sedimentary rocks of the south texas part of the gulf of mexico basin using salinity data rayleigh number calculations and numerical models previous studies speculated that free convection could account for high fluxes evidently required for diagenesis in the basin but low permeability shales are calculated to be a barrier to free convection in the study area salinity inversions occur either above or within the transition zone from hydropressures to overpressures the positioning of brines over less saline fluids provides a significant buoyancy force rayleigh number calculations and numerical simulations suggest that homogeneous shaly systems are unstable near the high end of the expected ranges of shale permeability 10 15 10 16 m2 numerical simulations show that the influx of brine into the permeable layers and permeability heterogeneity in the shales are both conducive to free convection simulations indicate that salinity driven free convection can occur at lower permeabilities 10 16 10 18 m2 that may approximate the permeabilities of shales in the zone of extreme overpressures over geologic time simple rayleigh numbers are inadequate to predict the occurrence of free convection in heterogeneous systems salinity driven free convection at depths in some large sedimentary basins such as the gulf of mexico may be more common than expected

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

crustal structure across the southwest newfoundland transform margin a seismic refraction survey providing deep crustal structure information of the continent ocean boundary across the south west newfoundland transform margin was carried out using large air gun sources and ocean bottom seismometer receivers continental crust 30 km thick beneath the southern grand banks p wave velocity 6 2 6 5 km s thins oceanward to a 25 km wide transition zone in the transition zone paleozoic basement of the grand banks 5 5 5 7 km s is replaced by a basement of oceanic volcanics and synrift sediments 4 5 5 5 km s seaward of the transition zone the crust is oceanic in character with a velocity gradient from 4 7 to 6 5 km s and a thickness of 7 8 km oceanic layer 3 is absent no significant thickness of intermediate velocity 7 km s material is present at the continent ocean transition indicating that no under plating of continental crust has taken place the continent ocean transition across the transform margin is much narrower than across rifted margins supporting the

iberian plate kinematics a jumping plate boundary between eurasia and africa the rotation of iberia and its relation to the formation of the pyrenees has been difficult to decipher because of the lack of detailed sea floor spreading data although several models have been proposed1 7 here we use detailed aeromagnetic measurements from the sea floor offshore of the grand banks of newfoundland to show that iberia moved as part of the african plate from late cretaceous to mid eocene time with a plate boundary extending westward from the bay of biscay when motion along this boundary ceased a boundary linking extension in the king s trough to compression along the pyrenees came into existence finally since the late oligocene iberia has been part of the eurasian plate with the boundary between eurasia and africa situated along the azores gibraltar fracture zone

rift tectonics on the passive continental margin off galicia spain abstract new migrated multichannel seismic profiles on the starved passive continental margin off galicia spain illustrate the tectonic style of the early cretaceous rifting event which preceded the separation of iberia and north america the main episode of rifting was preceded by other tensional episodes probably of earliest cretaceous and late jurassic age listric faulting associated with rifting was accompanied by large scale mass movements from the crests of the tilted blocks two subaerial erosional surfaces are visible one of late jurassic or earliest cretaceous age was affected by the final faulting episode the other was synchronous with the main phase of rifting and is present only on the upper part of the margin because the central trough was under deep water the significance of a ridge near the ocean continent boundary where altered lherzolite has been dredged and cored is discussed below the tilted blocks which include the basement rocks the seismic horizon s is visible as on the armorican margin its significance is also discussed

extensional tectonics and stratigraphy of hibernia oil field grand banks newfoundland the tectonic evolution of the grand banks involved several episodes of rifting and sequential development of the continental margin the dominant period of rifting and basin formation in the central grand banks was late callovian to aptian extensional failure of the central grand banks crust probably occurred along a shear zone that dipped gently to the west listric normal basin forming faults merge at depth with this detachment system the listric murre fault soles at 26 km 16 mi creating the very deep jeanne d arc basin cross basin transfer faults accommodated different amounts and rates of extension and resulted in the conspicuous offset pattern and funnel shaped geometry of the jeanne d arc basin the hibernia oil field is associated with one of these structural salients the stratigraphy of the jeanne d arc basin and the hibernia oil field is a direct response to this tectonic and structural framework large scale unconformity bounded sequences correspond to major plate reorganizations and zipper type opening of the atlantic ocean smaller scale adjustments to episodic extension are expressed in multiple stacking of depositional sequences the broad scale evolution of the synrift succession involved the early deposition of limestones and oil prone shales about 10 12 m y later rifting climaxed resulting in floods of coarse terrestrial material from the rift shoulders the characteristics of these depositional systems reflect a pervasive structural control affecting the distribution of facies tracts dip slip movement on basin bounding faults and c oss basin transfer fault trends were equally important a profound structural modification of the hibernia structure occurred in late barremian time when a major detachment developed within the basin fill this structural modification and associated antithetic and synthetic faulting in the hanging wall largely controlled reservoir distribution and the maturation migration and trapping of hydrocarbons

new ice core evidence for a volcanic cause of the a d 536 dust veil in new zealand human cryptosporidiosis demonstrates spring and autumn peaks of incidence with the spring peak being three times greater in magnitude than the autumn peak the imbalance between the two peaks is notable and may be associated with the high livestock density in new zealand in the summer and autumn the cryptosporidiosis rate was positively associated with temperatures in the current and previous month highlighting the importance of outdoor recreation to transmission no associations between spring incidence and weather were found providing little support for the importance of drinking water pathways imported travel cases do not appear to be an important factor in the aetiology of cryptosporidiosis in new zealand

direct transformation of tree ring measurements into palaeoclimate reconstructions in three dimensional space here we propose a new method for dendroclimatic reconstructions this direct reconstruction technique direct performs modelling of the age dependent association between climate and growth in thre

historic volcanism european dry fogs and greenland acid precipitation 1500 b c to a d 1500 historic dry fogs in europe acid precipitation in greenland and major explosive volcanic eruptions correlate well with each other between 1500 b c and a d 1500 european mediterranean and icelandic volcanic eruptions appear to be the source of at least five of the nine largest acidity signals found in greenland ice for this period between 152 b c and a d 43 eruptions of sulfur rich mount etna probably supplied about 15 percent of the smaller acidity signals

middle jurassic to early cretaceous igneous rocks along eastern north american continental margin late middle jurassic and early cretaceous mafic dikes sills flows and local volcaniclastic sediments are intercalated within continental shelf sediments from the baltimore canyon trough northward to the grand banks of newfoundland the igneous rocks on the eastern north american margin are mainly alkali basalts of intraplate affinity the late middle jurassic igneous activity was of short duration at about 140 ma and was restricted to georges bank where it led to construction of several volcanic cones the main period of igneous activity was concentrated at about 120 ma in the aptian barremian the activity consists of dike swarms in baltimore canyon occasional dikes on the scotian shelf and the growth of stratovolcanoes on the scotian shelf and grand banks younge dikes 95 ma also are present on the grand banks the major phases of igneous activity are synchronous with major regional tectonic events namely the initiation of rifting in the labrador greenland region 140 ma separation of continental plates between iberia and the grand banks 115 ma and separation of labrador and greenland continental plates 83 92 ma most of the igneous activity occurred along old fracture zones that appear to have been reactivated and or along fractures or fracture zones in the continental crust resulting from changes in stress patterns caused by plate motion the co occurrence of middle jurassic and early cretaceous volcanic activity on the continental slope off georges bank is evidence that a hot spot process alone cannot explain the linear character and age trends within the new england seamounts propagation of a fracture zone into oceanic crust more satisfactorily accounts for the age and distribution of the volcanism with regard to oil exploration on the continental margin care must be taken to properly identify igneous and volcaniclastic rocks on mechanical logs drill cuttings and cores reflection seismic profiles can be used to map the areal extent of sills flows and low angle dikes which commonly show distinctive seismic responses however steeply dipping dikes generally produce little if any seismic response isotopic age determinations of igneous rocks combined with biostratigraphic age determinations of adjacent strata are invaluable for stratigraphic correlation establishing chronology of seismic sequences and analysis of basin sedimentation and tectonic history

assessment of the record of the 1982 el chichon eruption as preserved in greenland snow variability in the so42 and cl time series for the 1980s from 12 shallow snow pits across the greenland ice sheet is used to evaluate the record of the 1982 el chichon eruption and the potential for recording a moderate northern equatorial eruption in a single greenland ice core composition of volcanic glass found in spring 1983 snow in one of the pits in the summit region matches that from el chichon glass thereby verifying the deposition of material from the eruption high na and cl concentrations in this same layer probably represent deposition of the reaction products of halite and h2so4 as observed in the stratosphere following the eruption these findings and the presence of a cl signal in five of the other pits indicate that the cl aerosol component of some eruptions has the potential to remain aloft for at least 1 year after the eruption some of these aerosols may be adsorbed onto tephra particles distinct so42 peaks that can be confidently linked to el chichon were found in only 50 of the records developed through subseasonal sampling however in several other pits an elevated baseline thought to represent more lengthy periods of el chichon aerosol deposition as opposed to deposition from a single snowfall event were observed smoothing of the original data by the calculation of annual so42 flux resulted in the presence of high flux values between 1982 and 1984 years thought to be affected by el chichon aerosol deposition in 9 of the 12 pits these results suggest that a single ice core from anywhere in greenland may record a signal from a northern equatorial eruption of magnitude similar to that of el chichon about 75 of the time this is despite the overall high levels of so2 4 deposition from anthropogenic sources that now make identification and quantification of the volcanic so2 portion of the record more difficult than obtaining the same data for preindustrial volcanism nevertheless composite records from all the pits sampled yielded stratospheric loading 20 mt and optical depth 0 13 estimates similar to stratospheric and satellite based measurements following the eruption equally high so42 concentration and flux values in snow from 1980 to 1982 reflect deposition from the series of middle to high northern latitude volcanic eruptions 1979 1981

climatic impact of the a d 1783 asama japan eruption was minimal evidence from the gisp2 ice core abstract assessing the climatic impact of the a d 1783 eruption of mt asama japan is complicated by the concur rent eruption of laki iceland estimates of the stratospheric loading of h2so 4 for the a d 1108 eruption of asama derived from the so42 time series in the gisp2 greenland ice core in dicate a loading of about 10 4 tg h2so 4 with a resulting stratospheric optical depth of 0 087 assuming sulfur emis sions from the 1783 eruption were only one third of the 1108 event yields a h2so 4 loading value of 3 5 tg and a strato spheric optical depth of only 0 029 these results suggest minimal climatic effects in the northern hemisphere from the 1783 asama eruption thus any volcanically induced cooling in the mid 1780s is probably due to the laki eruption introduction asama volcano 36 5on 138 9oe honshu japan has erupted many times over the last 1000 years aramaki 1956 1957 1963 but only two of these events have been large enough to definitely warrant a volcanic explosivity index

stress induced changes in plate density vail sequences epeirogeny and short lived global sea level fluctuations the stratigraphic record contains clear evidence that broad regions have experienced rapid changes in sea level unrelated to changes in glacial ice volume e g third and higher order vail sea level variations continental foundering we propose that many of these changes can be caused by stress induced changes in plate density stress changes produce significant changes in the density of the crust and lithosphere a point missed in previous investigations and propagate across even the largest plates in less than 30 000 years lithospheric plates interacting at existing boundaries can produce stress related density changes sufficient to cause several meters change in plate elevation these may account for many of the regressions and transgressions seen in the stratigraphic record the creation of new rifts could increase plate compression enough to cause 50 m of plate subsidence plate elevation changes of up to 200 m could result from increased plate compression during continental collisions a particularly enigmatic kind of rapid nonglacial global ngg sea level change is a coupled 50 m sea level fall regression and roughly equal rise transgression occurring in less than 106 years these couplets associated with black shales and marine extinctions can be explained by the elastic snapback attending the rapid formation of a new rift isostatic disequilibria along the new rift depresses the seafloor sufficiently to cause a 50 m fall in sea level mantle flow restores isostatic equilibrium along the rift axis and erases the fall in 60 000 years unusually intense hydrothermal circulation along the new rift during the snapback promotes anoxic bottom water conditions and deposition of black shales a rapid drop in global sea levels reduces the area of ocean bottom within the photic zone causing overpopulation food exhaustion and extinctions the connections between changes in plate stress and density have many implications perhaps the most far reaching is that tectonic events on the 3 4 of the globe that is covered by oceans as well as the 1 4 that is subaerial are recorded in sedimentary strata

lithospheric strength and rift migration controls on synrift stratigraphy and breakup unconformities at rifted margins examples from numerical models the atlantic and south china sea margins synrift stratigraphy and the distribution of breakup related erosional unconformities vary vastly between passive margins and cannot be explained by classical rifting models here we use numerical modeling to predict their spatiotemporal distribution we show that synrift stratigraphy mimics rift architecture which is controlled by lithospheric strength basinward rift migration during extension produces 1 oceanward younging syntectonic and posttectonic sequences 2 rift migration unconformities rmus predating breakup and 3 a breakup unconformity bu that only extends over the outermost margins since breakup is not linked with a sudden stress drop with small synrift sedimentation the rmus and bu laterally merge to form a margin wide unconformity in symmetric wide conjugate margins which arise for weak lithospheres such as the south china sea a long phase of distributed deformation with little subsidence results in early synrift sediment over most of the margins rmus merge into a single event that marks the subsequent focusing of deformation into a narrow breakup area which experiences short lived intense thinning and subsidence in asymmetric conjugate margins lateral rift migration transports shallowly deposited early synrift sediments from the narrow to the wide hyperextended margin leading to a condensed syntectonic sequence and a single bu in the narrow margin and a series of rmus in the wide one for very weak lower crusts lateral rift migration generates large synrift sag basins in the wide margin as in angola and congo margins our models resemble the observed margins tectonic diversity and may be used as templates for interpreting their distal unexplored areas

climate response to the samalas volcanic eruption in 1257 revealed by proxy records the eruption of samalas in indonesia in 1257 ranks among the largest sulfur rich eruptions of the common era with sulfur deposition in ice cores reaching twice the volume of the tambora eruption in 1815 sedimentological analyses of deposits confirm the exceptional size of the event which had both an eruption magnitude and a volcanic explosivity index of 7 during the samalas eruption more than 40 km 3 of dense magma was expelled and the eruption column is estimated to have reached altitudes of 43 km however the climatic response to the samalas event is debated since climate model simulations generally predict a stronger and more prolonged surface air cooling of northern hemisphere summers than inferred from tree ring based temperature reconstructions here we draw on historical archives ice core data and tree ring records to reconstruct the spatial and temporal climate response to the samalas eruption we find that 1258 and 1259 experienced some of the coldest northern hemisphere summers of the past millennium however cooling across the northern hemisphere was spatially heterogeneous western europe siberia and japan experienced strong cooling coinciding with warmer than average conditions over alaska and northern canada we suggest that in north america volcanic radiative forcing was modulated by a positive phase of the el nino southern oscillation contemporary records attest to severe famines in england and japan but these began prior to the eruption we conclude that the samalas eruption aggravated existing crises but did not trigger the famines

mystery cloud of ad 536 dry fogs appear in the atmosphere when large volcanic eruptions inject massive quantities of fine silicate ash and aerosol forming sulphur gases into the troposphere and stratosphere although the ash gravitationally settles out within weeks the aerosols spread around the globe and can remain suspended in the stratosphere for years because solar radiation is easily absorbed and backscattered by the volcanic particles a haziness in the sky and a dimming of the sun and moon are produced very dense and widespread dry fogs occur on the average once every few centuries1 3 the sizes and intensities of some of the largest of them before the modern scientific era have been estimated by several indirect methods2 4 5 the densest and most persistent dry fog on record was observed in europe and the middle east during ad 536 and 537 despite the earliness of the date there is sufficient detailed information to estimate the optical depth and mass of this remarkable stratospheric dust cloud the importance of this cloud resides in the fact that its mass and its climatic consequences appear to exceed those of any other volcanic cloud observed during the past three millenia although the volcano responsible remains a mystery a tropical location perhaps the volcano rabaul on the island of new britain papua new guinea can be tentatively inferred

mixed convection processes below a saline disposal basin abstract saline groundwater and irrigation drainage are commonly diverted and stored in both natural and artificial depressions throughout the murray darling basin of australia the disposal basin brines that are formed are often denser than ambient groundwater under certain conditions these dense brines may become unstable causing them to mix with groundwater over distances several orders of magnitude greater than due to diffusion alone a model is developed to study the mixed convection processes below a saline disposal basin located between a recharge and discharge zone numerical simulations are performed in cross section using the 2 d density dependent model sutra saturated unsaturated transport it is shown that the salt front movement is related to both the ability of the dense saline brines to mix convectively with underlying groundwaters rayleigh convection and the strength of the regional advective velocity both homogeneous and heterogeneous aquifer systems are studied and the effects of anisotropy are considered our numerical results suggest that the behaviour of a dense brine plume overlying less dense groundwater in a homogeneous porous medium depends on the magnitude of at least two non dimensional numbers a rayleigh number and modified peclet number defined in terms of basin scale hydrogeologic parameters including dispersion it is shown that the onset of gravitational instabilities and the formation of free convective cells begins when the magnitude of a non dimensional parameter combining the rayleigh and modified peclet number exceeds a certain critical value 1997 elsevier science b v all rights reserved

south pole ice core record of explosive volcanic eruptions in the first and second millennia a d and evidence of a large eruption in the tropics around 535 a d 1 a record of explosive eruptions over the last 1830 years reconstructed from a south pole antarctica ice core extends the coverage of volcanic history to the start of the first millennium a d the ice core dating by annual layer counting carries an uncertainty of 2 of the number of years from time markers with the largest dating error of 20 years at the bottom of the 182 m core several aspects of the methodology of detecting and quantifying volcanic sulfate signals in ice cores are examined in developing this record the new record is remarkably consistent with previous south pole records a comparison with records from several antarctica locations suggests that the south pole location is among the best for ice core reconstruction of volcanic records owing to the excellent preservation of volcanic signals at the south pole the relatively low and uniform sulfate background and the moderately high snow accumulation rates which allow for dating by annual layer counting a prominent volcanic event dated at 531 15 a d along with evidence from other records indicates that an unusually large eruption took place in the tropics and was probably responsible for the mystery cloud climate episode of 536 537 a d the date of 536 is suggested for a prominent volcanic signal that appears in the first half of the sixth century a d in ice cores which can in turn be used as a time stratigraphic marker in dating ice cores by annual layer counting or by computing average accumulation rates or layer thicknesses with such markers

radiative forcing and climate impact resulting from so2 injections based on a 200 000 year record of plinian eruptions along the central american volcanic arc we present for the first time a self consistent methodology connecting volcanological field data to global climate model estimates for a regional time series of explosive volcanic events using the petrologic method we estimated so2 emissions from 36 detected plinian volcanic eruptions occurring at the central american volcanic arc cava during the past 200 000 years together with simple parametrized relationships collected from past studies we derive estimates of global maximum volcanic aerosol optical depth aod and radiative forcing rf describing the effect of each eruption on radiation reaching the earth s surface in parallel aod and rf time series for selected cava eruptions are simulated with the global aerosol model maecham5 ham which shows a relationship between stratospheric so2 injection and maximum global mean aod that is linear for smaller volcanic eruptions 5 mt so2 and nonlinear for larger ones 5 mt so2 and is qualitatively and quantitatively consistent with the relationship used in the simple parametrized approximation potential climate impacts of the selected cava eruptions are estimated using an earth system model of intermediate complexity by rf time series derived by 1 directly from the global aerosol model and 2 from the simple parametrized approximation assuming a 12 month exponential decay of global aod we find that while the maximum aod and rf values are consistent between the two methods their temporal evolutions are significantly different as a result simulated global maximum temperature anomalies and the duration of the temperature response depend on which rf time series is used varying between 2 and 3 k and 60 and 90 years for the largest eruption of the cava dataset comparing the recurrence time of eruptions based on the cava dataset with the duration of climate impacts based on the model results we conclude that cumulative impacts due to successive eruptions are unlikely the methodology and results presented here can be used to calculate approximate volcanic forcings and potential climate impacts from sulfur emissions sulfate aerosol or aod data for any eruption that injects sulfur into the tropical stratosphere

extensional tectonics in the jeanne d arc basin offshore newfoundland implications for the timing of break up between grand banks and iberia

a revised age of ad 667 699 for the latest major eruption at rabaul the most recent major eruption at rabaul was one of the largest known volcanic events at this complex system having a volcanic explosivity index vei rating of 6 the eruption generated widespread pumice lapilli and ash fall deposits and ignimbrites of different types the total volume of pyroclastic material produced in the eruption exceeded 11 km3 and led to a new phase of collapse within rabaul caldera initial 14c dating of the event yielded an age of about 1400 years bp and the eruption became known as the 1400 bp eruption previous analyses of the timing of the eruption have sought to link it to events in ad 536 and ad 639 however we have re evaluated the age of the eruption using the bayesian wiggle match radiocarbon dating method and the eruption is now thought to have occurred in the interval ad 667 699 the only significant equatorial eruptions recorded in both greenland and antarctic ice during this interval are at ad 681 and ad 684 dates that coincide with frost rings in bristlecone pines of western usa in the same years definitively linking the rabaul eruption to this narrow age range will require identification of rabaul tephra in the ice records however it is proposed that a new working hypothesis for the timing of the most recent major eruption at rabaul is that it occurred in the interval ad 681 684

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

experimental models of hangingwall deformation in ramp flat listric extensional fault systems abstract the results of three experimental models of ramp flat extensional fault systems are presented the experiments were constructed from sand and mica layers to form an anisotropic sequence that would respond to the imposed deformation by both faulting and folding the geometry of the underlying detachment controls the deformation in the hangingwall sequence ramp flat detachment geometries produce an upper roll over with a crestal collapse graben system a ramp syncline and fold reverse fault zone and a lower roll over with a crestal collapse graben system short ramp geometries produce only minor perturbations in a larger roll over structure as extension progresses the crestal collapse systems migrate and superimpose earlier systems the models of ramp flat extensional structures produce templates for the interpretation of natural ramp flat listric fault systems

relationship between impact crater size and severity of related extinction episodes abstract how large must an extraterrestrial impact be to cause a peak episode of increased extinctions of life impact energies 3 107 mt tnt associated with terrestrial impact craters with final diameters 100 km seem to be required to generate significant widespread climatic effects from sub micron dust and soot in the atmosphere leading to a distinct extinction episode 15 extinction of marine genera impacts creating craters smaller than 100 km in final diameter in the 106 to 107 mt tnt range are capable of mostly regional destruction with minimal impact on global climate or biota these results are supported by the fact that the ages of the four known 100 km diameter craters of the last 260 my popigai chicxulub morokweng and manicouagan are all correlative with times of documented extinction episodes whereas smaller craters are not the largest crater the 180 km diameter chicxulub crater a 108 mt tnt event is associated with the more severe major mass extinction event 45 extinction of genera at the end of the cretaceous the percent species extinctions show a significant linear relationship with final crater diameter and impact energy the very large chicxulub impact lies close to the predicted curve of percent extinction versus impact crater diameter and energy but the low angle of impact an unusual composition of the target area with thick sediments rich in carbonates sulfates and organic material and a large excavated transient crater may have led to the generation of unusually large amounts of co2 widely distributed dust soot and sulfate aerosols and a uniquely severe impact related environmental disaster chicxulub may thus be the only large body impact associated with a major mass extinction in the phanerozoic target sensitivity may apply to large impacts into ocean crust having only a thin cover of organic poor and carbonate poor pelagic sediments and thus even large oceanic impacts which are still unknown may not produce enough dust soot and aerosols to cause environmental crises leading to global extinction peaks above background levels

subsidence history of australia s southern margin constraints on basin models the subsidence history of australia s southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation the first phase is characterized by rapid tectonic subsidence about 50 m m y which lasted for about 25 30 m y during the early cretaceous the period of rapid subsidence represents the rifting phase of margin development this early stage is correlated with associated rifting phenomena faulting and extension of the brittle upper crust formation of grabens and half grabens and the rapid accumulation of volcanogenic detritus otway group the amount of extension 20 50 is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin the model includes uniform extension and thinning of the continental lithosphere a prolonged extensional event followed by simple cooling horizontal and vertical conduction of heat and radioactive heat production by the crust the rift phase may have begun earlier but the most rapid and widespread subsidence occurred during the early cretaceous the end of rapid and widespread subsidence occurred during the early cretaceous the end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly about 90 ma adjacent to the continent ocean boundary cob the second stage of margin development is the post rift or drift phase a period of much slower rates of subsidence and relative tectonic quiescence subsidence during the post rift stage is modeled as cooling and contraction of extended continental lithosphere estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the cob estimates of extension from the geometry of basement blocks near the cob do not agree with the analysis from subsidence modeling upper crustal extension seems to record only a small proportion of total lithospheric extension alternative or supplementary mechanisms of extension are explored with emphasis on implications from a model that includes localized shear along a lithospheric detachment system

comment on mixed convection processes below a saline disposal basin by simmons c t narayan k a 1997 journal of hydrology 194 263 285

a 2000 year tree ring record of annual temperatures in the sierra nevada mountains tree ring data have been used to reconstruct the mean late season june through january temperature at a timberline site in the sierra nevada california for each of the past 2000 years long term trends in the temperature reconstruction are indicative of a 125 year periodicity that may be linked to solar activity as reflected in radiocarbon and auroral records the results indicate that both the warm intervals during the medieval warm epoch a d 800 to 1200 and the cold intervals during the little ice age a d 1200 to 1900 are closely associated with the 125 year period significant changes in the phase of the 125 year temperature variation occur at the onset and termination of the most recent radiocarbon triplet and may indicate chaotic solar behavior

response to the comment on mixed convection processes below a saline disposal basin simmons c t narayan k a 1997 journal of hydrology 194 263 285 by e holzbecher

radiocarbon and geologic evidence reveal ilopango volcano as source of the colossal mystery eruption of 539 40 ce abstract ilopango volcano el salvador erupted violently during the maya classic period 250 900 ce in a densely populated and intensively cultivated region of the southern maya realm causing regional abandonment of an area covering more than 20 000 km2 however neither the regional nor global impacts of the tierra blanca joven tbj eruption in mesoamerica have been well appraised due to limitations in available volcanological chronological and archaeological observations here we present new evidence of the age magnitude and sulfur release of the tbj eruption establishing it as one of the two hitherto unidentified volcanic triggers of a period of stratospheric aerosol loading that profoundly impacted northern hemisphere climate and society between circa 536 and 550 ce our chronology is derived from 100 new radiocarbon measurements performed on three subfossil tree trunks enveloped in proximal tbj pyroclastic deposits we also reassess the eruption magnitude using terrestrial el salvador guatemala honduras and near shore marine tbj tephra deposit thickness measurements together our new constraints on the age eruption size 43 6 km3 dense rock equivalent of magma magnitude 7 0 and sulfur yield 9 90 tg along with ilopango s latitude 13 7 n squarely frame the tbj as the major climate forcing eruption of 539 or 540 ce identified in bipolar ice cores and sourced to the tropics in addition to deepening appreciation of the tbj eruption s impacts in mesoamerica linking it to the major northern hemisphere climatic downturn of the mid 6th century ce offers another piece in the puzzle of understanding eurasian history of the period

continental break up mechanism lessons from intermediate and fast extension settings continental break up mechanisms vary systematically between slow and fast extension systems slow extension break up has been established from studies of the central atlantic european and adria margins this study focuses on the intermediate and fast cases from gabon and east india and draws from the interpretation of reflection seismic gravimetric and magnetic data interpretation indicates continental break up via continental mantle unroofing in all systems with modifications produced by magmatism in faster extension systems break up of the intermediate extension gabon system involves partial upper continental crustal decoupling from continental mantle whereas in the fast east coast india system decoupled and lower crustal regimes underwent upwarping in soggy zones in the footwalls of major normal faults usually upper crustal break up is affected by pre existing anisotropies which form systems of constraining rails for extending continental crust this modifies the local stress regimes they regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle different regions attain different amounts of upper crustal stretching prior to the break up the break up location is then controlled by the upper crustal energy balance principle of wound linkage by which the minimum physical work is performed for linking upper crustal wounds leading to successful upper crustal break up

response of the lithosphere to in plane force variations the earth s lithosphere deforms when subjected to vertical and horizontal tectonic forces because it is not perfectly rigid the deformation has both elastic flexural and inelastic brittle and ductile creep components we use the yield strength envelope formulation of a depth dependent rheology to predict how the lithosphere will deform in response to applied in plane force brittle deformation of the upper lithosphere is influenced by the distribution of existing faults and fractures in the crust and their orientation with respect to the applied tectonic force the sense of fault reactivation indicates the sign of the applied in plane force the induced flexural deformation is governed by the pre existing lithospheric deflection the effective elastic thickness of the lithosphere when the force was applied and the magnitude of the applied force using this framework we compare the predicted response of the lithosphere to in plane force variations with that observed in the central indian ocean and the jeanne d arc basin offshore newfoundland the brittle deformation in both regions is readily observed in seismic reflection data the upper oceanic crust in the central indian ocean is broken into blocks bounded by high angle reverse faults the dip of these basement faults and their orientation parallel to the marine magnetic lineations suggest that they represent a reactivated crustal fabric imparted during the formation of oceanic crust the reverse sense of fault reactivation is consistent with the north south compression determined from earthquake focal mechanisms in the jeanne d arc basin episodes of northwest southeast extension reactivated the basin bounding faults in a normal sense during late barremian early aptian and late aptian time in the late albian the same crustal blocks were reactivated and uplifted in response to north northeast in plane compression flexural deformation due to applied in plane force is observed only in the central indian ocean case where the oceanic crust and most of the overlying bengal fan sediments were deformed into broad east west trending undulations with wavelengths of 100 300 km and amplitudes up to 2 km in the jeanne d arc basin the flexural deformation which should accompany the fault reactivation is difficult to recognize from the preserved stratigraphy consequently brittle deformation within extensional basins which appears to be the most recognizable response of the lithosphere to in plane force variations obscures the effects of the flexural deformation however it is this flexural deformation that has been postulated as a tectonic alternative to glacial eustasy in explaining the 3rd order sea level cycles because it is difficult to recognize the effects of the flexural deformation in the preserved stratigraphy we surmise that it is not a viable alternative to glacial eustasy in generating either regional or global unconformities instead we propose that the flexural and brittle deformation due to applied in plane force may locally enhance or diminish the effects of other processes responsible for unconformity generation

fan deltas and braid deltas varieties of coarse grained deltas two types of coarse grained deltas are recognized fan deltas and braid deltas fan deltas are gravel rich deltas formed where an alluvial fan is deposited directly into a standing body of water from an adjacent highland they occupy a space between the highland usually a fault bounded margin and the standing body of water in contrast braid deltas here introduced are gravel rich deltas that form where a braided fluvial system progrades into a standing body of water braid deltas have no necessary relationship with alluvial fans as exemplified by fluvioglacial braid deltas braid deltas have previously been classified as fan deltas even though the geomorphic and sedimentologic settings of the two systems can be vastly different braid deltas are a common present day geomorphic feature and are abundant in the geological record fan deltas and braid deltas can be distinguished in the rock record by distinctive subaerial components of these depositional systems the shoreline and subaqueous components of both are similar fan delta sequences have a subaerial component that is an alluvial fan facies comprising interbedded sheetflood debris flow and braided channel deposits fan deltas produce small a few tens of square kilometres wedge shaped bodies of sediment commonly displaying high variability in paleocurrent patterns and abrupt changes in facies the deposits are generally very coarse grained with large out sized clasts very poorly sorted matrix rich polymictic heterolithic partially cemented by penecontemporaneous carbonate and have low porosity and permeability braid deltas in contrast have a subaerial component consisting entirely of braided river or braidplain facies their deposits display better sorting roundness and clast orientation than do fan delta sediments they lack a muddy matrix they display size grading and bar migration they commonly have a sheet geometry with high lateral continuity tens to hundreds of square kilometres and they exhibit moderate to high porosity and permeability valuable paleogeographic and tectonic information concerning the proximity of highlands and major fault zones may be misinterpreted or lost if these two coarse grained deltaic systems are not differentiated

magmatism at the west iberia non volcanic rifted continental margin evidence from analyses of magnetic anomalies we discuss the magmatic development of the west iberia non volcanic rifted continental margin in the north atlantic ocean so called non volcanic rifted continental margins are characterized by a lack of syn rift magmatism and are considered to be largely amagmatic however this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously we concentrate our attention on the recently described zone of exhumed continental mantle zecm to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the zecm leading to the onset of seafloor spreading we use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide angle seismic experiments presented elsewhere forward and inverse modelling of a sea surface magnetic anomaly chart and of surface and deep towed magnetometer profiles shows that anomalies within the zecm differ in trend amplitude and source type from those in the adjacent oceanic crust and thinned continental crust the zecm anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin we interpret such bodies as syn extensional intrusions that increase in volume oceanward they eventually merge in the vicinity of a margin parallel basement peridotite ridge to give rise to a continuous crust that records reversals in the earth s magnetic field from the time of anomaly m4 n m5 r i e to mark the onset of seafloor spreading we find no evidence for anomalies formed by seafloor spreading at either slow or ultraslow rates before m5 r 128 ma

mid to late holocene climate evolution of the lake telmen basin north central mongolia based on palynological data abstract palynological and sedimentological data from lake telmen in north central mongolia permit qualitative reconstruction of relative changes in moisture balance throughout the mid to late holocene the climate of the atlantic period 7500 4500 yr ago was relatively arid indicating that lake telmen lay beyond the region of enhanced precipitation delivered by the expanded asian monsoon maximum humidity is recorded between 4500 and 1600 cal yr b p during the subboreal 4500 2500 yr ago and early subatlantic 2500 yr present periods additional humid intervals during the medieval warm epoch 1000 1300 a d or 950 650 ago and the little ice age 1500 1900 a d or 450 50 yr b p demonstrate the lack of long term correlation between temperature and moisture availability in this region a brief aridification centered around 1410 cal yr b p encompasses a decade of cold temperatures and summer frost between a d 536 and 545 1414 1405 yr b p inferred from records of mongolian tree ring widths these data suggest that steppe vegetation of the lake telmen region is sensitive to centennial and decadal scale climatic perturbations

thermomechanical implications of sediment transport for the architecture and evolution of continental rifts and margins erosion and deposition redistribute mass as a continental rift evolves which modifies crustal loads and influences subsequent deformation surface processes therefore impact both the architecture and the evolution of passive margins here we use coupled numerical models to explore the interactions between the surface crust and lithosphere this interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink if transport is efficient there are two possible outcomes 1 faulting within the zone of extension is longer lived and has larger offsets this implies a reduction of the number of faults and the width of the proximal domain 2 efficient transport of sediment leads to significant deposition and hence thermal blanketing this will induce a switch from brittle to ductile deformation of the upper crust in the distal domains the feedbacks between these two outcomes depend on the extension history the underlying lithospheric rheology and the influence of submarine deposition on sediment transport high erosion sedimentation during early faulting leads to abrupt crustal necking while intermediate syntectonic sedimentation rates over distal deep submarine hotter crust leads to unstructured wide distal domains in models where rheological conditions favor the formation of asymmetric conjugate margins only subaerial transport of sediments into the distal domains can increase conjugate symmetry by plastic localization these models suggest that passive margin architecture can be strongly shaped by the solid earth structure sea level and climatic conditions during breakup

major optical depth perturbations to the stratosphere from volcanic eruptions pyrheliometric period 1881 1960 a detailed chronology of major stratospheric dust veils from 1881 to 1960 has been constructed by searching the primary literature for relevant observational data of various kinds especially pyrheliometry data from 23 observing sites in both hemispheres have been reduced in a rigorous fashion to yield a table of pyrheliometric optical depth perturbations as a function of year month and latitude band to convert to visual reference wavelengths the tabular entries should be multiplied by a time dependent factor that is somewhat greater than unity ten measurable volcanic dust veils have been established in this manner and have been analyzed as to their generation transport decay and total mass these clouds arose from eruptions of krakatau 1883 an unidentified volcano 1890 soufriere and pelee 1902 santa maria 1902 ksudach 1907 katmai 1912 puyehue 1921 paluweh 1928 komagatake 1929 and quizapu 1932 other turbidity indicators have also been used in the analysis including starlight extinction purple twilight glows color of sun and moon polarization of blue sky light bishop s ring around the sun and dark lunar eclipses global stratospheric aerosol loadings have been computed from the peak turbidities agreement with the aerosol masses derived from polar ice cores is good in the important cases of krakatau santa maria and after a major correction of the ice core value katmai during the long period 1881 1992 about 80 of all stratospheric aerosols generated by the largest sulfur producing eruptions were emplaced during the two short time intervals 1883 1902 and 1982 1991 the long term 1881 1992 average annual production rate of stratospheric so2 from the largest eruptions was 0 8 tg yr 1 about half the average rate since 1981 implications of the principal results are discussed

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

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

on a test case for density dependent groundwater flow and solute transport models the salt lake problem like any other computer model density dependent mathematical groundwater models must be verified to ascertain that they accurately represent the physics implied by a governing set of equations very few test cases for density dependent groundwater numerical models exist as such there is still a need for new and more robust tests of these modeling codes in this paper a numerical model of an idealized evaporating salt lake is produced using the two dimensional density dependent saturated unsaturated transport sutra model the results of which are compared with an equivalent laboratory hele shaw cell system developed by wooding et al 1997a b evaporation results in dense brine overlying less dense fluid which is hydrodynamically unstable and leads to downward convection of salt fingers or plumes a comparison of experimental and numerical plume growth shows good spatial and temporal agreement the numerically generated plume pattern is sensitive to changes in random noise level applied just below the evaporation surface that serves as a trigger for the growth of instabilities experimental plume patterns were best matched with a noise level corresponding to 1 of the total salinity difference between boundary layer and background fluid concentrations at saturation in a second comparison the stream function based finite difference model described by wooding et al 1997a b which differs significantly in principle from sutra is shown after revision to give good spatial and temporal agreement with experimental results this test for density dependent groundwater models appears to be the most comprehensive and detailed available to date

switch of mesozoic extensional tectonic style in the lusitanian basin portugal insights from magnetic fabrics abstract magnetic fabrics in the lusitanian basin show extreme variability in the orientation of strain axes during part of its mesozoic extensional stage a detailed study of anisotropy of magnetic susceptibility ams was done in triassic and upper jurassic units in the onshore part of the basin in the triassic samples the magnetic lineation shows a dominant nw se orientation perpendicular to the main normal fault set and also to the extension directions inferred from the analysis of brittle mesostructures for the late jurassic period the ams analysis indicates a dominant n s to ne sw extension moreover these main directions are complicated by perpendicular solutions co existing in the same sites comparison with syn sedimentary faults reveals the difficulty in interpreting ams data in these settings although magnetic stability for the obtained solutions can be tracked through different mineralogical composition for the sampled rocks interpretation of the obtained fabrics through a triassic radial extension regime is consistent with axes switching and local re orientation of strain axes close to major normal faults related to the opening of the northern atlantic during the jurassic

thinned and subsided continental crust on the rifted margin of eastern canada crustal structure thermal evolution and subsidence history summary seismic refraction measurements were carried out using ocean bottom seismometers over foundered continental crust in orphan basin and flemish pass on the continental margin north east of newfoundland tau p travel time inversion synthetic seismogram analysis and conventional layered model calculations were applied to the data the results show that these outer regions of the margin are underlain by thinned continental crust with a total depth to the m discontinuity of about 22 km there are two main crustal layers with p velocities of about 6 1 and 7 0 km s 1 which appear to be homogenous and separated by sharp interfaces these are overlain by a layer in which p velocities are about 5 5 km s 1 this is interpreted to be precambrian or palaeozoic basement mesozoic and cenozoic sediments cover the basement rocks and are over 4 km thick the results imply that crustal thinning to about 50 per cent of the original crustal thickness occurred the gravity anomaly data show that the thin crust has a maximum horizontal extent of about 450 km from the ocean continent boundary near orphan knoll landward to the outer continental shelf the subsidence history and thermal evolution of the region was computed assuming that the observed thinning is produced by horizontal extension of the lithosphere it is suggested that extension can only satisfy the observed crustal structure and elevation of the margin during the rift phase if more extension took place in the lower lithosphere than in the upper lithosphere the computed subsidence is compared to the observed subsidence and the total amounts of subsidence are similar however the shape of the observed subsidence curves measured in deep exploratory wells differs significantly from the predicted subsidence assuming cooling began when final continental breakup occurred in the late cretaceous the temperature distribution within the lithosphere due to extension may be related to the flexural rigidity of the plate as a function of both time and position across the margin therefore it is suggested that the response of the lithosphere to sediment loading and the large amplitude of the gravity shelf edge anomalies are directly related to the thermal history of the region

reconciling multiple ice core volcanic histories the potential of tree ring and documentary evidence 670 730 ce chronologically secure volcanic event histories are important for improving our understanding of volcano climate responses and securing ice core chronologies we present an exploratory case study that attempts to reconcile the chemical fingerprints of major volcanism in greenland ice cores for the years 670 730 ce this period experienced considerable volcanic perturbation with multiple volcanic signals registered in all 8 greenland ice core datasets studied including some comparable in magnitude to the great 1815 tambora eruption but reconciling signals with divergent dating between datasets presents a number of challenges to further our understanding of the volcanic history of this period frost rings tree ring growth width and density minima from europe siberia and china are considered together with western european near eastern and chinese documentary evidence these reveal a striking sequence of co occurring inter regional climatic extremes and social crises we further examine a recently proposed 7 year adjustment to the gicc05 chronology used by many greenland ice cores and find that this results in a more coherent volcanic history for these years and an improved agreement with tree ring and written records three major volcanic events can now be identified dated to 681 684 686 and 706 707 with several moderate events further contributing to a sustained climate perturbation reflected in prolonged temperature reductions in high resolution reconstructions

the role of small scale extensional faulting in the evolution of basin geometries an example from the late palaeozoic petrel sub basin northwest australia abstract during continental extension the kinematic collapse of the hangingwall of major normal faults and the subsequent isostatic response imposes a strong control on the evolving basin geometry the interaction of the flexural wavelength and the magnitude and location of faults may allow the development of basin geometries which deviate from a classic half graben style particularly if some of this deformation is below the scale of observation commonly seismic reflection data in particular the development of a lateral partitioning between large and small scale faults within a basin may exert a significant control on the resulting basin geometry using the petrel sub basin in northwest australia as an example it is demonstrated that an extensional basin geometry consisting of a classic half graben can be overprinted by a significant sag geometry which can be related to the lateral offset of sub resolution faulting this lateral partitioning and resulting basin geometry may also have an application to other extensional basins particularly if a mechanism is present to allow this partitioning of fault styles to develop in the petrel sub basin this has been related to the presence of older basement features of the halls creek mobile zone beneath the axis of the basin however a comparison with physical models also suggests that this may be applicable to basins formed by oblique rifting a component of which may also be present within the petrel sub basin particularly if this is imposed upon a weak zone or suture within the upper mantle

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

the case for significant numbers of extraterrestrial impacts through the late holocene when astronomers tell us that there should have been numerous impacts from space during the last five millennia when impact craters exist on land and more impacts can be assumed over the oceans why are historians archaeologists and palaeoecologists not diligently seeking evidence for these impacts and their effects this article reviews just some of the relevant evidence for impacts in turn this suggests that ablation material background material from space and micro tektites should all be present in ocean cores ice cores peat and lake sediments it seems that almost no efforts have been made to find evidence that might link to the known crater fields or to identify and date periods of enhanced cosmic activity the question must be why copyright 2007 john wiley sons ltd

upper triassic lower jurassic salt basin southeast of the grand banks abstract a grid of multichannel seismic reflection mcs profiles has been used to delineate and map a large apparently tectonically controlled basin and associated diapiric province beneath the continental slope and rise southeast of the grand banks of newfoundland the depocenter for which we propose the name salar basin is defined both landward and seaward by pronounced hinges in acoustic basement it is approximately 400 km long 35 70 km wide and covers an area of approximately 20 000 km 2 the southern end of the basin is marked by the intersection of the southeast newfoundland ridge with the south bank high on the grand banks and the northern end is bounded by the southwestern margin of flemish cap the salar basin generally parallels the much smaller carson basin which lies beneath the grand banks immediately to the west and it merges with this basin north of 45 22 n based upon correlation with contiguous deposits sampled by drilling in the carson basin we infer piercements filling the salar basin to be mobilized late triassic early jurassic evaporites probably predominantly halite a consequent early mesozoic age for the salar basin implies a multi stage rifting history for the newfoundland basin as has been documented for the adjacent grand banks initial extension which probably formed the salar basin s bounding basement hinge zones occurred in late triassic early jurassic time and was associated with rifting of north america from africa and extension in western europe the presence of early mesozoic evaporites in a basin southeast of the grand banks suggests development of a significant but shallow and restricted seaway between the grand banks and the iberian peninsula probably connected northward with coeval salt provinces of northwestern europe a second rift phase related to separation of iberia and the grand banks affected the grand banks and salar basin in late jurassic to early cretaceous time after a period of epeirogenic subsidence during the early and middle jurassic uplift associated with this extension led to the development of the widespread u avalon unconformity across the grand banks the salar basin and the newfoundland basin out to the j anomaly and may have induced the first halokinesis in the salar basin the second rift phase culminated in the initiation of seafloor spreading in mid early cretaceous aptian time at a location now marked by the j anomaly

dendrochronology raises questions about the nature of the ad 536 dust veil event new evidence is presented which shows the response of a wide grid of tree ring chronologies to the dust veil of ad 536 the effects of which were historically recorded in the mediterranean and eastern asia the tree ring evidence particularly that from european oaks quercus shows a two stage response which raises questions about whether only a single massive eruption was involved the observations from precisely dated tree ring series set the agenda for the interpretation of forthcoming ice core evidence across the ad 530s and 540s