new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

millennial scale fluctuations of the european ice sheet at the end of the last glacial and their potential impact on global climate abstract reconstructing northern hemisphere ice sheet oscillations and meltwater routing to the ocean is important to better understand the mechanisms behind abrupt climate changes to date research efforts have mainly focused on the north american laurentide ice sheets lis leaving the potential role of the european ice sheet eis and of the scandinavian ice sheet sis in particular largely unexplored using neodymium isotopes in detrital sediments deposited off the channel river we provide a continuous and well dated record for the evolution of the eis southern margin through the end of the last glacial period and during the deglaciation our results reveal that the evolution of eis margins was accompanied with substantial ice recession especially of the sis and simultaneous release of meltwater to the north atlantic these events occurred both in the course of the eis to its lgm position i e during heinrich stadial hs 3 and hs2 31 29 ka and 26 23 ka respectively and during the deglaciation i e at 22 ka 20 19 ka and from 18 2 0 2 to 16 7 0 2 ka that corresponds to the first part of hs1 the deglaciation was discontinuous in character and similar in timing to that of the southern lis margin with moderate ice sheet retreat from 22 5 0 2 ka in the baltic lowlands as soon as the northern summer insolation increase from 23 ka and an acceleration of the margin retreat thereafter from 20 ka importantly our results show that eis retreat events and release of meltwater to the north atlantic during the deglaciation coincide with amoc destabilisation and interhemispheric climate changes they thus suggest that the eis together with the lis could have played a critical role in the climatic reorganization that accompanied the last deglaciation finally our data suggest that meltwater discharges to the north atlantic produced by large scale recession of continental parts of northern hemisphere ice sheets during hs could have been a possible source for the oceanic perturbations i e amoc shutdown responsible for the marine based ice stream purge cycle or so called he s that punctuate the last glacial period

glacier fluctuations in the southern peruvian andes during the late glacial period constrained with cosmogenic 3he the occurrence of pronounced climate reversals during the last glacial termination has long been recognised in palaeoclimate records from both hemispheres and from high to low latitudes accurate constraint of both the timing and magnitude of events such as the younger dryas and antarctic cold reversal is vital in order to test different hypotheses for the causes and propagation of abrupt climate change however in contrast to higher latitude regions well dated records from the tropics are rare and the structure of late glacial tropical climate remains uncertain as a step toward addressing this problem we present an in situ cosmogenic 3 he surface exposure chronology from nevado coropuna southern peru documenting a significant fluctuation of the ice margin during the late glacial period ten tightly clustered ages from a pair of moraines located halfway between the modern glacier and the last glacial maximum terminus range from 11 9 to 13 9ka and give an arithmetic mean age of 12 8 0 7ka 1s these data constitute direct evidence for a readvance or prolonged stillstand of glaciers in the arid andes of southwestern peru copyright 2011 john wiley sons ltd

timing and structure of mega sacz events during heinrich stadial 1 a substantial strengthening of the south american monsoon system sams during heinrich stadials hs points toward decreased cross equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time scales in order to better constrain the exact timing and internal structure of hs1 over tropical south america we assessed two precisely dated speleothem records from central eastern and northeastern brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial atlantic during hs1 we recognize at least two events of widespread intensification of the sams across the entire region influenced by the south atlantic convergence zone sacz at 16 11 14 69 kyr b p and 18 1 16 66 kyr b p labeled as hs1a and hs1c respectively separated by a dry excursion from 16 66 to 16 11 kyr b p hs1b in view of the spatial structure of precipitation anomalies the widespread increase of monsoon precipitation over the sacz domain was termed mega sacz

structure of the southern falkland islands continental shelf initial results from new seismic data abstract acquisition of a modern marine dataset has revealed the internal structure of sedimentary basins to the south of the falkland islands for the first time the new data reveal a variety of structural styles and indicate the presence of a sedimentary section ranging from 2 to 10 km in thickness developed in three linked sub basins 1 the eastern part of the malvinas basin lies south west of the islands a prominent regional unconformity developed throughout the area marks the top of gently folded rocks of probable palaeozoic age and their cover of late jurassic volcanics to the north west of the island the underlying strata approach the surface in a structural high which forms a prominent gravity low and may record the continuation of the palaeozoic basin exposed on the islands themselves strata of probable mesozoic and tertiary age rest on the unconformity throughout the area and show a regional southward dip these deposits increase in thickness south westwards towards the depocentre of the malvinas basin midway between the islands and argentina 2 the south falkland basin and burdwood bank lie south of the islands thicknesses increase towards an area of southward dipping extensional fault blocks within the south falkland basin downdip these are buried beneath a prograding and north vergent stack of blind and emergent thrusts at the northern margin of the burdwood bank which is a structural highly lying a shallow water south of the islands thrust loading has resulted in the development of a foreland basin north of the thrust front complex antiformal structures are associated with the thrusting whereas a shallow sedimentary basin developed on the crest of the burdwood bank is interpreted as a piggy bank basin riding on and sourced dominantly from within the thrust pile 3 the falkland plateau basin lies to the east and south east of the islands and is characterized by a varied and extensive mesozoic to tertiary succession of submarine fan deposits thickening eastwards into deeper water igneous intrusives deep in the section may correlate with jurassic volcanic rocks present within the malvinas basin to the west or may record younger rifting associated with the opening of the south atlantic these initial results indicate considerable structural heterogeneity in this part of the falkland islands continental shelf in the light of the hydrocarbon discoveries already made further west significant economic potential is likely stratigraphic traps may occur at regional unconformities and within submarine fan deposits whereas potential structural traps include mesozoic extensional fault blocks and thrust and sub thrust structures generated by tertiary and quaternary compression

magnetic investigations of the west hawk deep bay and clearwater impact structures canada due to the effects of erosion tectonism and burial impact structures are often obscured or destroyed geophysical methods are increasingly being used in detecting the signatures of impact structures while gravity lows associated with impact structures are well understood associated magnetic anomaly lows are not in this study drill cores from three canadian impact structures were analyzed for rock magnetic properties and mineralogy in order to explain the magnetic anomaly lows associated with these structures samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility ams and natural remanent magnetization nrm parameters drill cores from the twin impact craters of the clearwater structure exhibited different nrm characteristics and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques the difference noted in their nrm characteristics was attributed to the acquisition of a viscous remanent magnetization vrm at depth in clearwater east at all three structures both magnetic susceptibilities and remanent magnetizations are well below regional values in impact generated breccias melt rocks shocked crystalline rocks and in postimpact sedimentary infill the processes of brecciation alteration shock and infill by nonmagnetic sediments contribute to the development of the magnetic lows however a significant contribution to the observed magnetic anomalies was found by first order forward modelling to arise from basement rocks beneath the impact structures this zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact induced transient stress wave traveling away from the point of impact

geophysical research on the kardla impact structure hiiumaa island estonia the 4 km wide and 500 m deep circular kardla impact structure in hiiumaa island estonia of middle ordovician age 455 ma is buried under upper ordovician and quaternary sediments to constrain the geophysical models of the structure petrophysical properties such as magnetic susceptibility natural remanent magnetization nrm density electrical conductivity porosity and p wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area the results were used to interpret the central gravity anomaly of 3 mgal and the magnetic anomaly of 100 nt and also the surrounding weak positive anomalies revealed by high precision survey data the unshocked granitic rocks outside the structure have a mean density of 2630 kgm 3 their shocked counterparts have densities of 2400 kgm 3 at a depth of 500 m increasing up to 2550 kgm 3 at a depth of 850 m porosity and electrical conductivity decrease but p wave velocity increases as density increases away from the impact point thus the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for kardla as in many other impact structures the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks a new geophysical and geological model of the kardla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks together with structural data from numerous boreholes an important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores

timing constraints on deformation history of the sudbury impact structure herein we present new high resolution seismic images of the sudbury impact structure acquired across the sudbury igneous complex and its environs which provide evidence for the relative timing of the deformation events that reshaped the initial sudbury structure the seismic images show that the lower unit of the sudbury basinal fill sediments the onwatin argillite is penetrated by a set of blind imbricated thrusts whereas the overlying chelmsford turbidites are unaffected by faulting we interpret this observation to mean that the deposition of the chelmsford sediments postdates the latest major deformation of the sudbury structure suggesting that the uniform paleocurrent trends observed in the chelmsford turbidites are not related to the initial shape of the sudbury structure

the geological evolution of the falkland islands continental shelf the falkland islands are surrounded by four major sedimentary basins the falkland plateau basin to the east the south falkland basin to the south the malvinas basin to the west and the north falkland basin to the north the four main basins appear to have formed initially as triassic through earliest cretaceous extensional rifts associated with the break up of gondwana a valanginian end to rifting was followed by thermal sag there is evidence of cenozoic uplift in at least the north falkland basin possibly coincident with andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the scotia sea there is no evidence from offshore seismic and gravity magnetic data to support interpretations that the falkland islands have rotated clockwise through up to 180 during gondwana separation with the exception of the south falkland basin all the major basins probably underwent initially more or less east west extension and had a similar orientation to adjacent south american and western southern african basins the falkland basins probably shared a similar geological history with the offshore southern african and south american basins

geophysical evidence for a large impact structure on the falkland malvinas plateau a large roughly circular structural basin is recognised on the falkland malvinas plateau to the nw of west falkland gran malvina island s 51 00 w 62 00 the basin seen in seismic reflection profiles and evident as a large negative gravity anomaly has a diameter of 250 km the age of the basin is estimated to be late palaeozoic it is completely buried by younger sediments and has no topographic expression on the sea floor we propose that the basin and geophysical anomalies especially the combination of a large circular negative gravity anomaly with a rim of positive anomalies and a marked circular series of positive magnetic anomalies in the same area may be best explained by the presence of a large buried impact structure

diatom productivity in the equatorial pacific ocean from the last glacial period to the present a test of the silicic acid leakage hypothesis 1 the silicic acid leakage hypothesis salh suggests that during glacial periods unused silicic acid escaped the southern ocean into the equatorial oceans causing an ecological shift favoring diatoms relative to coccolithophorids and a drawdown of co2 the 230th normalized opal fluxes and 231pa 230th ratios were measured in eleven equatorial pacific cores to reconstruct diatom productivity over the past 30 kyr and to test the salh holocene spatial patterns of opal flux are strongly correlated with 231pa 230th ratios and with satellite estimates of primary productivity down core opal flux records do not support the salh but show holocene opal burial to exceed that of the late glacial period by 35 or 2 8 gt opal kyr across the equatorial pacific this suggests that the salh may not account for lower atmospheric co2 levels during the late glacial however the data do support results from previous studies that invoke increased el nino like conditions during this time

salinity bias on the foraminifera mg ca thermometry correction procedure and implications for past ocean hydrographic reconstructions 1 mg ca in foraminiferal calcite has recently been extensively used to estimate past oceanic temperatures here we show however that the mg ca temperature relationship of the planktonic species globigerinoides ruber is significantly affected by seawater salinity with a 1 psu change in salinity resulting in a 1 6 c bias in mg ca temperature calculations if not accounted for such a bias could lead for instance to systematic overestimations of mg ca temperatures during glacial periods when global ocean salinity had significantly increased compared to today we present here a correction procedure to derive unbiased sea surface temperatures sst and 18osw from g ruber tmg ca and 18of measurements this correction procedure was applied to a sedimentary record to reconstruct hydrographic changes since the last glacial maximum lgm in the western pacific warm pool while uncorrected tmg ca data indicate a 3 c warming of the western pacific warm pool since the lgm the salinity corrected sst result in a stronger warming of 4 c

structural outer rim of chesapeake bay impact crater seismic and bore hole evidence nine seismic reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the chesapeake bay impact crater the rim is manifested as a 90 km diameter ring of terraced normal fault blocks which forms a 320 m 1200 m high rim escarpment the top of the rim escarpment is covered by a 20 m 30 m thick ejecta blanket the escarpment encircles a flat floored annular trough which is partly filled with an 250 m thick breccia lens exmore breccia the exmore breccia overlies a 200 m 800 m thick interval of slumped sedimentary megablocks which in turn rests on crystalline basement rocks all postimpact strata upper eocene to quaternary sag structurally into the annular trough and most units also thicken as they cross the rim into the crater postimpact compaction and subsidence of the exmore breccia have created extensive normal faulting in overlying strata

adjustment of the coupled ocean atmosphere system to a sudden change in the thermohaline circulation 1 the adjustment of the coupled ocean atmosphere system to a fresh water anomaly in the north atlantic ocean is investigated using a coupled gcm in response to the anomaly the atlantic thermohaline circulation thc collapses and the associated reduction in northward heat transport causes sea surface temperatures sst to fall over the north atlantic and to increase over the tropical south atlantic atmospheric feedbacks are excited as soon as the sst anomalies reach sufficient magnitude a key stage in the adjustment process occurs 4 6 years after the perturbation is introduced when a significant sst dipole develops in the tropical atlantic this dipole causes a southward shift of the itcz and leads in year 7 to the triggering of an el nino event it is concluded that atmospheric feedbacks could spread globally the influence of a sudden change in the thc much more quickly and efficiently than could ocean processes alone

stratigraphy and sedimentology of the permian of the falkland islands lithostratigraphic and palaeoenvironmental links with south africa comparison of the facies ichnology and palaeocurrent patterns of the permian of the falkland islands and the dwyka ecca and beaufort part groups in south africa supports the hypothesis that the falkland islands lay east of south africa in a rotated position prior to continental break up key sections in the falklands nw lafonia and the eastern cape ecca pass would have lain about 200 km apart in permian time in the absence of accurate chonostratigraphic and biostratigraphic information from the permian of the falkland islands emphasis is placed on lithostratigraphic comparison the permian successions in both areas pass from diamictite through basin floor sediments including organic rich shale rhythmites and turbidites mud dominated delta front deposits culminating in channel deposits of a generally subaqueous delta top overlie the basin floor sediments points of close lithostratigraphic comparison are 1 the diamictites have identical appearance facies and compatible palaeocurrents 2 glacial sections are overlain by organic rich black shales 3 k bentonites occur in basin floor successions 4 presence of basin floor turbidite sandstone units with interbedded rhythmite units and a distinctive umfolozia undichna ichnofauna 5 coarsening up delta front sequences are overlain by fining up channel sandstone facies 6 there is a close petrographic similarity indicating derivation of sandstones from a contemporaneous volcanic arc the permian successions of the two areas form parts of the fill of the eastern end of the main karoo basin and were subject to the same influences of tectonic development sediment derivation eustatic variation and climatic change

late palaeozoic to mesozoic structural evolution of the falkland islands a displaced segment of the cape fold belt the falkland islands lie on a displaced crustal block presently forming part of the south american plate the islands possess two roughly orthogonal structural grains the relative chronology of which is here unequivocally established for the first time d1 structures form a southerly verging permo triassic age fold belt striking e w to wnw ese which represents a displaced segment of the gondwanian orogenic belt the strike swing from e w to wnw ese is coincident with a decrease in deformation intensity toward the west on west falkland d1 folds are superimposed by the early mesozoic d2 ne sw trending hornby mountains anticline producing localized kilometre scale type i and iii fold interference patterns the hornby mountains anticline is interpreted to be the result of dextral transpressive reactivation of a pre existing ne sw basement fault if the falkland islands are rotated 180 the structure of the d1 fold belt and tectono sedimentary features of the permian age lafonian supergroup display a remarkable correlation with the structural style and tectonic evolution of the eastern cape fold belt of south africa our data therefore support a 180 tectonic rotation of the falkland islands and reaffirm their pre gondwana break up position adjacent to the southeast coast of south africa in such a reconstruction the falkland islands constrain the easterly extension and eventual lateral termination of the cape fold belt

tectonics of complex crater formation as revealed by the haughton impact structure devon island canadian high arctic the results of a systematic field mapping campaign at the haughton impact structure have revealed new information about the tectonic evolution of mid size complex impact structures these studies reveal that several structures are generated during the initial compressive outward directed growth of the transient cavity during the excavation stage of crater formation 1 sub vertical radial faults and fractures 2 sub horizontal bedding parallel detachment faults and 3 minor concentric faults and fractures uplift of the transient cavity floor toward the end of the excavation stage produces a central uplift compressional inward directed deformation results in the duplication of strata along thrust faults and folds it is notable that haughton lacks a central topographic peak or peak ring the gravitational collapse of transient cavity walls involves the complex interaction of a series of interconnected radial and concentric faults while the outermost concentric faults dip in toward the crater center the majority of the innermost faults at haughton dip away from the center complex interactions between an outward directed collapsing central uplift and inward collapsing crater walls during the final stages of crater modification resulted in a structural ring of uplifted intensely faulted sub vertical and or overturned strata at a radial distance from the crater center of 5 0 6 5 km converging flow during the collapse of transient cavity walls was accommodated by the formation of several structures 1 sub vertical radial faults and folds 2 positive flower structures and chaotically brecciated ridges 3 rollover anticlines in the hanging walls of major listric faults and 4 antithetic faults and crestal collapse grabens oblique strike slip i e centripetal movement along concentric faults also accommodated strain during the final stages of readjustment during the crater modification stage it is clear that deformation during collapse of the transient cavity walls at haughton was brittle and localized along discrete fault planes separating kilometer size blocks

regional north american gravity and magnetic anomaly correlations summary correlation of gravity and magnetic anomalies combined with other geological and geophysical data is useful for enhancing the quality of geological interpretation of potential anomaly fields maps produced by equivalent point source inversion are used to investigate visual spatial correlations of surface free air gravity and pogo satellite magnetic anomalies and regional heat flow and tectonic data for north america and adjacent marine areas a quantitative analysis of regional potential field anomaly correlations at satellite elevations is also considered utilizing poisson s theorem in a moving window linear regression between derivatives of the anomalous gravity and magnetic fields an inverse relationship is observed between long wavelength gravity and magnetic anomalies over continental terrain negative gravity and positive magnetic anomalies are areas characterized by relatively thick crust and high magnetization an example is a prominent magnetic high which corresponds to a trend of gravity minima extending from the anadarko basin to the cincinnati arch negative magnetic and positive gravity anomalies characterize thinner crust and regions of higher heat flow such as the cordillera of north and central america and specifically the yellowstone geothermal region although gravity and magnetic anomalies over oceanic areas show poor correlation the sign of the statistical correlation generally is positive 1 crustal correlation considerations the utility of long wavelength gravity anomalies covering several degrees of surface area for geological analysis is well recognized magnetic anomalies which have significant energy in wavelengths of several hundred kilometres also have been identified as originating from within the earth s crust and perhaps the uppermost mantle the geological interpretation of these regional anomalies is hindered by the effect of anomaly superposition and source ambiguity which is inherent to the analysis of potential fields an approach to minimizing these limitations especially in continental terrains is to evaluate the correlation between anomalous gravity and magnetic fields the basis of correlation analysis is the hypothesis commonly validated in continental areas that variations in lithology and physical properties of the crystalline crust are reflected

a cosmogenic 10be chronology for the local last glacial maximum and termination in the cordillera oriental southern peruvian andes implications for the tropical role in global climate abstract resolving patterns of tropical climate variability during and since the last glacial maximum lgm is fundamental to assessing the role of the tropics in global change both on ice age and sub millennial timescales here we present a10be moraine chronology from the cordillera carabaya 14 3 s a sub range of the cordillera oriental in southern peru covering the lgm and the first half of the last glacial termination additionally we recalculate existing 10be ages using a new tropical high altitude production rate in order to put our record into broader spatial context our results indicate that glaciers deposited a series of moraines during marine isotope stage 2 broadly synchronous with global glacier maxima but that maximum glacier extent may have occurred prior to stage 2 thereafter atmospheric warming drove widespread deglaciation of the cordillera carabaya a subsequent glacier resurgence culminated at 16 100 yrs followed by a second period of glacier recession together the observed deglaciation corresponds to heinrich stadial 1 hs1 18 000 14 600 yrs during which pluvial lakes on the adjacent peruvian bolivian altiplano rose to their highest levels of the late pleistocene as a consequence of southward displacement of the inter tropical convergence zone and intensification of the south american summer monsoon deglaciation in the cordillera carabaya also coincided with the retreat of higher latitude mountain glaciers in the southern hemisphere our findings suggest that hs1 was characterised by atmospheric warming and indicate that deglaciation of the southern peruvian andes was driven by rising temperatures despite increased precipitation recalculated 10be data from other tropical andean sites support this model finally we suggest that the broadly uniform response during the lgm and termination of the glaciers examined here involved equatorial pacific sea surface temperature anomalies and propose a framework for testing the viability of this conceptual model

tectonostratigraphy and the petroleum systems in the northern sector of the north falkland basin south atlantic abstract the north falkland basin represents one of the frontier areas for hydrocarbon exploration in the south atlantic this study presents the results of new subsurface mapping using 2d seismic data in the north of the falkland islands offshore area which has delineated a series of discrete grabens northwards of the main north falkland basin referred collectively to as the northern sector of the north falkland basin nnfb six regionally significant seismic reflectors are interpreted within this data dividing the sedimentary fill into six tectonostratigraphic packages including early syn rift late syn rift transitional unit early post rift middle to late post rift and a sag unit structural interpretation of the 2d seismic data has led to the definition of four north south orientated depocentres namely 1 the eastern graben largest of the depocentres 20 km wide by 45 km long reaching depths of 3 km 2 the eastern graben splay a smaller depocentre 10 km wide by 20 km long reaching depths of 2 2 5 km 3 the western graben splay the smallest depocentre 5 km in width and 20 km long with a basin depth of 2 km and 4 the newly defined phyllis graben which is 13 km wide and 30 km long with a basin depth of 3 km a network of nw se and ne sw trending faults controls the development of these grabens separated by a western eastern and intra basin high these grabens represent a northern continuation of the northern falkland basin to the south hydrocarbon discoveries to the south of this study area e g sea lion casper beverley zebedee isobel deep and liz confirm a working petroleum system adjacent to the northern sector this study has identified a number of seismic anomalies including amplitude brightening events which potentially correspond to an extension of this petroleum system indicating active migration pathways the main targets in terms of hydrocarbon interest in the northern sector are likely to be stratigraphically trapped hydrocarbon accumulations contained within vertically amalgamated turbidite fan sandstone reservoirs deposited within the early post rift a second yet to be tested syn rift play in which the trapping geometries are structural and the reservoirs are fluvial sandstones is also identified

rapid last deglacial thinning and retreat of the marine terminating southwestern greenland ice sheet abstract marine terminating outlet glaciers are a major source of modern ice loss from the greenland ice sheet gris but their role in gris retreat during the last deglaciation is not well constrained here we develop deglacial outlet glacier retreat chronologies for four regions in southwest and south greenland to improve understanding of spatial variations in centennial to millennial scale ice loss under a warming climate we calculate 10be surface exposure ages of boulders located in fjords near the towns of qaqortoq paamiut nuuk and sisimiut our northernmost study site sisimiut deglaciated earliest at 18 ka to 15 ka with an average thinning rate of 0 1 0 3 m yr 1 inland retreat from sisimiut to the modern ice margin took 7 ka at an average retreat rate of 15 20 m yr 1 a 10be dated moraine 25 km from the modern gris margin deposited at 8 ka suggests a possible ice margin still stand but this does not change overall retreat rates after retreat from the small coastal sisimiut fjords the gris margin was mainly land terminating in this region in contrast earliest exposure occurred at 12 ka near qaqortoq and 11 10 ka near nuuk and paamiut with ice thinning at rates of 0 2 0 3 m yr 1 to instantaneous within measurement uncertainty ice retreat inland through the extensive nuuk paamiut and qaqortoq fjord systems to near modern ice margins occurred in

3d seismic reflection mapping of the silverpit multi ringed crater north sea the first comprehensive mapping of the silverpit multi ringed crater is presented based on five adjoining 3d seismic surveys that together provide 78 216 km2 areal coverage of the structure with spatial and depth sampling of 10 m the top cretaceous surface shows an excavated crater 3 km in diameter surrounded by 10 15 concentric ring structures out to a radius of 9 4 km seismic sequence stratigraphy and drilled wells indicate the lower age bound is a disconformity spanning 74 54 ma the upper bound is eocene termination of concentric ring fault growth ca 45 ma the inner rings are extensional faults defining rotated fault blocks that would be considered part of the crater in an impact structure interpretation giving a final impact crater diameter of 8 km the outer rings show a surprising variation in style from graben in the west to compressional structures in the east this may be explained by postcratering gravity spreading on easterly dipping intra cretaceous detachments within the crater the top cretaceous seismic reflector is disrupted and blocky but defines a central uplift with a slight crestal depression below this uplift the prominent base cretaceous seismic reflector shows jurassic strata rising into a cone 300 m high and 1 km wide characterized by radial faults this central uplift within the crater may structurally balance the inner ring fault blocks via a slumping model but we also identify low seismic velocity in the uplifted jurassic so fracturing may have contributed to growth of the central peak at a radius of 2 5 km base cretaceous shows a dish shaped depression that we interpret as marking either the extent of slumping toward the crater at this structural level or the limit of shockwave induced porosity collapse seismic reflectors above the top cretaceous excavated crater show differential compaction during the eocene accommodated by drape structure plus two conical faults that define narrow ring graben and represent the final phase of crater related tectonics

the chesapeake bay bolide impact a convulsive event in atlantic coastal plain evolution abstract until recently cenozoic evolution of the atlantic coastal plain has been viewed as a subcyclical continuum of deposition and erosion marine transgressions alternated with regressions on a slowly subsiding passive continental margin their orderly succession modified mainly by isostatic adjustments occasional appalachian tectonism and paleoclimatic change this passive scenario was dramatically transformed in the late eocene however by a bolide impact on the inner continental shelf the resultant crater is now buried 400 500 m beneath lower chesapeake bay its surrounding peninsulas and the continental shelf east of delmarva peninsula this convulsive event and the giant tsunami it engendered fundamentally changed the regional geological framework and depositional regime of the virginia coastal plain and produced the following principal consequences 1 the impact excavated a roughly circular crater twice the size of rhode island 6400 km2 and nearly as deep as the grand canyon 1 3 km deep 2 the excavation truncated all existing ground water aquifers in the target area by gouging 4300 km3 of rock from the upper lithosphere including proterozoic and paleozoic crystalline basement rocks and middle jurassic to upper eocene sedimentary rocks 3 synimpact depositional processes including ejecta fallback massive crater wall failure water column collapse and tsunami backwash filled the crater with a porous breccia lens 600 1200 m thick at a phenomenal rate of 1200 m hr the breccia lens replaced the truncated ground water aquifers with a single 4300 km3 reservoir characterized by ground water 1 5 times saltier than normal sea water chlorinities as high as 25 700 mg l 4 a structural and topographic low created by differential subsidence of the compacting breccia persisted over the crater at least through the pleistocene in the depression are preserved postimpact marine lithofacies and biofacies upper eocene lower oligocene lower miocene not known elsewhere in the virginia coastal plain 5 long term differential compaction and subsidence of the breccia lens spawned extensive fault systems in the postimpact strata many of these faults appear to reach the bay floor and may be potential hazards for motion sensitive structures in population centers around chesapeake bay near surface fracturing and faulting generated by the impact shock may extend as far as 90 km from the crater rim 6 having never completely filled with postimpact sediments the sea floor depression over the crater appears to have predetermined the location of chesapeake bay 7 as large impact craters are principal sources for some of the world s precious metals it is reasonable to expect that metal enriched sills dikes and melt sheets are present in the inner basin of the crater in addition to these specific consequences the crater and the convulsive event that produced it have widespread implications for traditional interpretations of certain structural and depositional features of the atlantic coastal plain particularly in southeastern virginia

relative timing of last glacial maximum and late glacial events in the central tropical andes whether or not tropical climate fluctuated in synchrony with global events during the late pleistocene is a key problem in climate research however the timing of past climate changes in the tropics remains controversial with a number of recent studies reporting that tropical ice age climate is out of phase with global events here we present geomorphic evidence and an in situ cosmogenic 3 he surface exposure chronology from nevado coropuna southern peru showing that glaciers underwent at least two significant advances during the late pleistocene prior to holocene warming comparison of our glacialgeomorphic map at nevado coropuna to mid latitude reconstructions yields a striking similarity between last glacial maximum lgm and late glacial sequences in tropical and temperate regions exposure ages constraining the maximum and end of the older advance at nevado coropuna range between 24 5 and 25 3 ka and between 16 7 and 21 1 ka respectively depending on the cosmogenic production rate scaling model used similarly the mean age of the younger event ranges from 10 to 13 ka this implies that 1 the lgm and the onset of deglaciation in southern peru occurred no earlier than at higher latitudes and 2 that a significant late glacial event occurred most likely prior to the holocene coherent with the glacial record from mid and high latitudes the time elapsed between the end of the lgm and the late glacial event at nevado coropuna is independent of scaling model and matches the period between the lgm termination and late glacial reversal in classic mid latitude records suggesting that these events in both tropical and temperate regions were in phase

size and morphology of the chicxulub impact crater the chicxulub impact in mexico has been linked to the mass extinction of species at the end of the cretaceous period from seismic data collected across the offshore portion of the impact crater the diameter of the transient cavity is determined to be about 100 km this parameter is critical for constraining impact related effects on the cretaceous environment with previous estimates of the cavity diameter spanning an order of magnitude in impact energy the offshore seismic data indicate that the chicxulub crater has a multi ring basin morphology similar to large impact structures observed on other planets such as venus

a revised position for the rotated falkland islands microplate the early stages of transform margin formation are associated with crustal fragmentation and block rotation the restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo an example considered here is the falkland plateau this is located adjacent to the agulhas falkland fracture zone and its westernmost province is the falkland islands microcontinent the position of the plateau and the islands prior to gondwana break up remains contentious this study integrates seismic reflection and gravity data to propose a revised position of the falkland islands microcontinent constrained by 1 the presence of a mega decollement controlling the gondwanide orogen described north of the falkland islands and underneath south africa and the outeniqua basin and 2 the similar architecture of fault networks mapped north of the islands and in the northernmost outeniqua basin this revised position requires a re evaluation of the timing and rate of rotation of the falkland islands microcontinent and affects the expected crustal architecture adjacent to the islands our model yields rotation rates between 5 5 and 8 ma 1 and two potential times for rotation and predicts more unstretched crust beneath the basin east of the falkland islands than previous models

abrupt shift in subsurface temperatures in the tropical pacific associated with changes in el nino radiocarbon 14c content of surface waters inferred from a coral record from the galapagos islands increased abruptly during the upwelling season july through september after the el nino event of 1976 sea surface temperatures ssts associated with the upwelling season also shifted after 1976 the synchroneity of the shift in both 14c and sst implies that the vertical thermal structure of the eastern tropical pacific changed in 1976 this change may be responsible for the increase in frequency and intensity of el nino events since 1976

sequence of events during the last deglaciation in southern ocean sediments and antarctic ice cores variation depends on the age model as 14 c determinations cannot be obtained for the time interval of 29 5 14 5 ka assuming a constant sedimentation rate for this interval our data suggest that sea ice and nutrient changes at about 19 ka b p lead the increase in atmospheric pco2 by approximately 2000 years our diatom based sea ice record is in phase with the sodium record of the vostok ice core which is related to sea ice cover and similarly leads the increase in atmospheric co2 if gas exchange played a major role in determining glacial to interglacial co2 variations then a delay mechanism of a few thousand years is needed to explain the observed sequence of events otherwise the main cause of atmospheric pco2 change must be sought elsewhere rather than in the southern ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4267 oceanography general paleoceanography 4845 oceanography biological and chemical nutrients and nutrient cycling 4870 oceanography biological and chemical stable isotopes 9325 information related to geographic region atlantic ocean keywords biogenic opal stable isotopes diatoms ird last glacial southern ocean

extensive mis 3 glaciation in southernmost patagonia revealed by cosmogenic nuclide dating of outwash sediments abstract the timing and extent of former glacial advances can demonstrate leads and lags during periods of climatic change and their forcing but this requires robust glacial chronologies in parts of southernmost patagonia dating pre global last glacial maximum glgm ice limits has proven difficult due to post deposition processes affecting the build up of cosmogenic nuclides in moraine boulders here we provide ages for the rio cullen and san sebastian glacial limits of the former bahia inutil san sebastian bi ssb ice lobe on tierra del fuego 53 54 s previously hypothesised to represent advances during marine isotope stages mis 12 and 10 respectively our approach uses cosmogenic 10be and 26al exposure dating but targets glacial outwash associated with these limits and uses depth profiles and surface cobble samples thereby accounting for surface deflation and inheritance the data reveal that the limits formed more recently than previously thought giving ages of 45 6 ka 139 9 14 3 for the rio cullen and 30 1 ka 45 6 23 1 for the san sebastian limits these dates indicate extensive glaciation in southern patagonia during mis 3 prior to the well constrained but much less extensive mis 2 glgm limit this suggests the pattern of ice advances in the region was different to northern patagonia with the terrestrial limits relating to the last glacial cycle rather than progressively less extensive glaciations over hundreds of thousands of years however the dates are consistent with mis 3 glaciation elsewhere in the southern mid latitudes and the combination of cooler summers and warmer winters with increased precipitation may have caused extensive glaciation prior to the glgm

investigation of a self organization model for beach cusp formation and development a recent numerical investigation of self organization werner and fink 1993 suggests that the feedback process between currents and sediment response can result in self organized patterns and can be used to predict beach cusp formation and spacing a similar model based on self organization is tested here in order to understand the processes occurring during beach cusp formation and development to evaluate the sensitivity toward the parameters used and to examine how the model might relate to field observations results obtained confirm the validity of the self organization approach and its capacity to predict beach cusp spacing with values in fair agreement with the available field measurements with most of the input parameters primarily affecting the rate of the process rather than the final spacing however changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior cusp spacing tends to change with time and cusp regularity shows large long term variations cusps are found to be accretionary in the swash zone and in agreement with most observations mean flows are horn divergent over developed topography simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed results indicate that preexisting large amplitude cusps are destroyed if their spacing is substantially different from that expected under self organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach these findings support the hypothesis that self organization is a robust mechanism for beach cusp formation

tropical pacific climate response to obliquity forcing in the pleistocene 1 marine proxy records of pleistocene seawater temperature and productivity in the tropical pacific ocean vary over a 41 000 year period that has been attributed to earth s obliquity cycle the proxy records are paradoxical both because obliquity has a small effect on low latitude insolation and because tropical seawater temperature and productivity were anticorrelated with obliquity insolation forcing in this study we investigate the response of the tropical pacific climate to obliquity forcing using a coupled ocean atmosphere model to reconcile the proxy records with climate theory two glacial and two modern simulations were completed with extreme high and low axial tilts of 24 5 and 22 2 in response to an increase in axial tilt tropical sea surface temperatures decrease by as much as 0 8 c because of the local reduction in insolation subsurface water temperatures in the eastern and central equatorial pacific increase by nearly 1 c anomalous heating through high obliquity forcing also generates dynamical responses that weaken mean annual midlatitude westerlies and subtropical trade winds contributing to a 20 reduction in the subtropical gyre circulation analyses using a lagrangian transport model indicate that low latitude subsurface warming is due to a reduction in heat export from the tropics and the advection and ventilation of anomalously warm south pacific extratropical waters through the thermocline circulation the model s response to obliquity is consistent with pleistocene proxy data that indicate the tropical walker circulation and thermocline slope were not strongly influenced by changes in axial tilt the model results also support the hypothesis that earth s obliquity influences climate through its control on meridional insolation gradients

the bosumtwi meteorite impact structure ghana a magnetic model abstract a magnetic model is proposed for the bosumtwi meteorite impact structure in ghana africa this relatively young 1 07 ma structure with a diameter of 10 5 km is exposed within early proterozoic birimian tarkwaian rocks the central part of the structure is buried under postimpact lake sediments and because of lack of drill cores geophysics is the only way to reveal its internal structure to study the structure below and beyond the lake a high resolution low altitude 70 m airborne geophysical survey across the structure was conducted which included measurements of the total magnetic field electromagnetic data and gamma radiation the magnetic data show a circumferential magnetic halo outside the lakeshore 12 km in diameter the central north part of the lake reveals a central negative magnetic anomaly with smaller positive side anomalies north and south of it which is typical for magnetized bodies at shallow latitudes a few weaker negative magnetic anomalies exist in the eastern and western part of the lake together with the northern one they seem to encircle a central uplift our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt rich suevite bodies petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites suevites have a higher magnetization and have low densities and high porosities compared to the target rocks in suevites the remanent magnetization dominates over induced magnetization koenigsberger ratio 3 preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the jaramillo normal polarity epoch this interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake the reverse polarity remanence component superimposed on the normal component is probably acquired during subsequent reverse polarity events