effects of shock induced incongruent melting within earth s crust the case of biotite melting introductiontectonic processes that involve partialmelting anatexis have deeply a ectedthe rheology and geodynamics of theearths lithosphere over the past 4 ga melting and extraction of large vol umes of granitic magma from the deepcrust give rise to an upper crustenriched in felsic silica rich mineralsand heat producing radioactive ele ments e g rudnick 1995 petfordet al 2000 the volume of extractedmelt is dependent on pressure temperature conditions source com position and most importantly theabundance of hydrous minerals which break down via incongruentreactions with quartz and feldspar toproduce granitic liquids e g stevenset al 2007 numerous experimentalstudies have shown that 20 30 ofincongruent melt can be produced attemperatures readily attained in thedeep crust during orogenesis or un derplating by basaltic magmas e g ellis and thompson 1986 vielzeufand holloway 1988 patino douceand beard 1995 less understood however are the processes of partialmelting caused by meteorite impacts under modern day geothermal gradi ents 13 30 ckm

precambrian reidite discovered in shocked zircon from the stac fada impactite scotland terrestrial impact events have had a profound influence on earth s geological geochemical and biological evolution however the record of precambrian impacts is poorly constrained due to the dynamic nature of plate tectonics erosion and deposition of younger rocks that may destroy or cover the evidence here we report the first precambrian occurrence of the rare mineral reidite zrsio4 within grains of shocked zircon in the ca 1 18 ga stac fada member stoer group northwestern scotland the reidite preserved as 2 m wide lamellae is unambiguous evidence of shock pressures in excess of 30 gpa and confirms the impact origin for the stac fada deposit the reidite lamellae are locally deformed and sites of deformation record its decomposition to baddeleyite zro2 and amorphous silica the first natural example of this transformation the findings demonstrate that reidite and baddeleyite may form and be transported in high energy ejecta without physical or chemical breakdown and are stable during sedimentary diagenesis and low grade metamorphism thus reidite may be preserved over time scales exceeding 1 b y establishing the use of reidite within detrital shocked zircon from precambrian strata as a viable and valuable means of recognizing and characterizing ancient terrestrial impact events

exchange through the bab el mandab during the last 10 years there have been a number of important developments in the understanding of the exchange flow in the bab el mandab new and improved estimates of the fluxes of mass heat and salt through the strait have been made directly from oceanographic measurements and indirectly from air sea flux data hydraulic theory has been applied to explain the seasonal variations in the exchange flow and in particular the three layer summer circulation the effects of wind stress friction and seasonal sea level variability on hydraulic control are addressed in this paper analyses of oceanographic data have failed to provide direct evidence of hydraulic control but they have highlighted the importance of tidal scale variability and differences between real continuously stratified fluids and idealised layer flows often used in hydraulic theory other studies have investigated the coupling between the flow in the strait and the circulation within the red sea

a 300 kyr high resolution aridity record of the north african continent new oxygen isotope data and a high resolution carbonate record of a core from the northwest african margin are presented in combination with geochemical zr rb ratio al ba organic carbon and biogenic opal fluxes and sedimentological median grain size parameters to show variations in african continental aridity wind strength and upwelling productivity the carbonate record is diluted by the supply of aeolian dust from the arid saharan region of north africa and is seen to vary with a characteristic sawtooth pattern indicating rapid increases in dust flux followed by gradual declines spectral analysis of the carbonate profile and comparison with specmap stacked oxygen isotope and etp solar radiation profiles shows that the precessional 23 kyr insolation induced african climate system is decoupled from global ice volume climate changes operating on a 100 kyr frequency detailed studies of glacial terminations i ii and iii confirm the presence of rapid short timescale climatic variations in aridity

aerial crystallization of hematite in impact bombs from the araguainha astrobleme mato grosso central brazil optical and scanning electron microscope sem observations of hematite impact bombs from the araguainha astrobleme in central brazil reveal an aerodynamically stable microstructure which is interpreted to reflect the aerial crystallization of hypervelocity melt globules derived from fe rich sedimentary target rocks the bombs are composed of an aggregate of preferentially oriented conic units with different degrees of packing giving rise to porous and massive hematite varieties these varieties generally occupy the front and rear of the bombs respectively quartz fragments with planar deformation features are common in the porous domains and represent ejected solid particles captured by the bombs during cooling the absence of typical contorted shapes in these bombs reflects the lack of spin during their ballistic trajectory the bombs are enriched in rh 4 ru ir and pd 5 and pt 30 relative to the indigenous level of target sediments this suggests the presence of a projectile derived component in their constitution single composition oxidic bombs pure hematite and pure silica predominate in this occurrence indicating that a mechanism of segregation has operated in the ejected melt volumes

late quaternary glacial interglacial cyclicity models of the red sea the benthic foraminiferal faunas from samples of two piston cores retrieved along a north south transect in the red sea were studied the northern core was collected during meteor cruise m 31 2 while the southern one was collected during the sonne cruise 121 benthic foraminiferal faunas from both sites exhibit large variability with respect to density diversity species composition and assemblages combined with stable oxygen and carbon isotope records of planktic and benthic foraminifera one hundered thirty benthic foraminiferal species were identified in the investigated cores the faunal data set of the northern core was reduced to five assemblages factors while the southern one was reduced to four assemblages all assemblages were ranked according to their ecological significance besides relative abundance of major benthic foraminiferal suborders textulariina agglutinating foraminifera miliolina and rotaliina in addition to infaunal epifaunal relative abundance were used as paleoenvironmental proxies allowing the reconstruction of past changes in deep water salinity ventilation and organic carbon fluxes at the sea floor four distinct glacial interglacial cycle during the past 380 kyr have been recognized in the red sea the identified four cycles reveal deviation in deep sea ecosystem between the northern and southern red sea in the northern red sea salinity fluctuations productivity and deep water ventilation and formation had the major impact on benthic foraminiferal pattern corresponding to glacial interglacial cycles and glacio eustatic sea level changes coupled with the impact of mediterranean climate regime on the otherhand in the southern red sea region the oscillation trend of benthic foraminiferal pattern within the glacials and interglacials stages indicates a high frequency environmental alternation this alternation is consistent with the extent of ne monsoonal wind that controls the intensity and extension of the productivity which in turn determine organic matter fluxes and oxygen level at the sea floor

paleoceanographic record of the last glacial interglacial cycle in the red sea and gulf of aden abstract hydrographic conditions in the red sea are strongly controlled by regional climatic conditions and the restricted water exchange with the gulf of aden indian ocean in turn intermediate and deep water masses in the gulf of aden reflect the nature of red sea outflow planktonic and benthic foraminiferal faunas and the lithology and mineralogy of red sea and gulf of aden sediments were studied in order to evaluate changes in water mass conditions in the two basins during the last 18 000 years the results indicate that changes in the local climate and circulation regime during this period resulted in significant hydrographic changes in surface and bottom waters in the red sea and intermediate waters in the gulf of aden during the last glacial maximum high aridity and glacio eustatic sea level lowering resulted in surface and bottom water salinities in the red sea which were considerably higher than at present in the gulf of aden glacial surface salinities and temperatures remained similar to holocene values while surface water productivity may have been lower surface salinities in the red sea decreased after 18 000 years b p as the connection between the red sea and gulf of aden was reestablished however bottom water salinities remained high until at least 11 000 years b p pluvial conditions during deglaciation enhanced the density difference between surface and bottom waters in the red sea and resulted in a decrease in the oxygen content of the bottom waters hydrographic conditions in the western gulf of aden reflect a change in the properties of outflowing red sea water during deglaciation

a pressure temperature phase diagram for zircon at extreme conditions abstract hypervelocity impact processes are uniquely capable of generating shock metamorphism which causes mineralogical transformations and deformation that register pressure p and temperature t conditions far beyond even the most extreme conditions created by terrestrial tectonics the mineral zircon zrsio4 responds to shock deformation in various ways including crystal plasticity twinning polymorphism e g transformation to the isochemical mineral reidite formation of granular texture and dissociation to zro2 sio2 which provide robust thermobarometers that record different extreme conditions the importance of understanding these material processes is two fold first these processes can mobilize and redistribute trace elements and thus be accompanied by variable degrees of resetting of the u pb system which is significant for the use of zircon as a geochronometer second some features described herein form exclusively during shock events and are diagnostic criteria that can be used to confirm the hypervelocity origin of suspected impact structures we present new p t diagrams showing the phase relations of zrsio4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints we present case studies to illustrate zircon microstructures formed in extreme environments and present electron backscatter diffraction data for grains from three impact structures mistastin lake of canada ries of germany and acraman of australia that preserve different minerals and microstructures associated with different shock conditions for each locality we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the p t history we outline a conceptual framework for a zircon based approach to extreme thermobarometry that incorporates both direct observation of high p and high t phases as well as inferences for the former existence of phases from orientation relationships in recrystallised products a concept we refer to here as phase heritage this new approach can be used to unravel the pressure temperature history of zircon bearing samples that have experienced extreme conditions such as rocks that originated in the earth s mantle and those shocked during impact events on earth and other planetary bodies

impact shocked zircons discovery of shock induced textures reflecting increasing degrees of shock metamorphism abstract textural effects specifically characteristic of shock metamorphism in zircons from impact environments have not been reported previously however planar deformation features pdf due to shock metamorphism are well documented in quartz and other mineral grains from these same environments an etching technique was developed that allows sem visualization of pdf and other probable shock induced textural features such as granular polycrystalline texture in zircons from a variety of impact shock environments these textural features in shocked zircons from k t boundary distal ejecta form a series related to increasing degrees of shock that should correlate with proportionate resetting of the u pb isotopic system

geochronological constraints on the age of a permo triassic impact event u pb and 40ar 39ar results for the 40 km araguainha structure of central brazil abstract impact cratering has been a fundamental geological process in earth history with major ramifications for the biosphere the complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination hampering the assessment of the effects of an individual event in the geological record we demonstrate the utility of a multi chronometer approach in our study of samples from the 40 km diameter araguainha impact structure of central brazil samples of uplifted basement granite display abundant evidence of shock deformation but u pb ages of shocked zircons and the 40ar 39ar ages of feldspar from the granite largely preserve the igneous crystallization and cooling history mixed results are obtained from in situ 40ar 39ar spot analyses of shocked igneous biotites in the granite with deformation along kink bands resulting in highly localized partial resetting in these grains likewise spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material the age of the glass itself and post impact devitrification the timing of crater formation is better assessed using samples of impact generated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization granular aggregates of neocrystallized zircon form a cluster of ten u pb ages that yield a concordia age of 247 8 3 8 ma the possibility of pb loss from this population suggests that this is a minimum age for the impact event the best evidence for the age of the impact comes from the u th pb dating of neocrystallized monazite and 40ar 39ar step heating of three separate populations of post impact inclusion rich quartz grains that are derived from the infill of miarolitic cavities the 206pb 238u age of 254 5 3 2 ma 2 error and 208pb 232th age of 255 2 4 8 ma 2 error of monazite together with the inverse 18 point isochron age of 254 10 ma mswd 0 52 for the inclusion rich quartz grains yield a weighted mean age of 254 7 2 5 ma 0 99 2 error for the impact event the age of the araguainha crater overlaps with the timing of the permo triassic boundary within error but the calculated energy released by the araguainha impact is insufficient to be a direct cause of the global mass extinction however the regional effects of the araguainha impact event in the parana karoo basin may have been substantial

shaking a methane fizz seismicity from the araguainha impact event and the permian triassic global carbon isotope record abstract the late permian and early triassic periods are marked by large fluctuations in the carbon isotope record but the source s of the disturbance to the global carbon cycle and the link to the end permian mass extinction are widely debated this contribution explores the possible isotopic effects of an impact event into the hydrocarbon rich rocks of the parana karoo basin recent u pb and 40ar 39ar dating of the 40 km araguainha impact structure of central brazil reveals an age of 254 7 2 5 ma 2 error for this event the calculated energy 105 106 mt of tnt equivalent released by this impact is less than threshold values of 107 108 mt tnt equivalent for global mass extinctions thus the araguainha crater is unlikely to have been the cause of the end permian biotic crisis however the combined seismic effects from the impact itself and the post impact collapse of the 20 25 km diameter transient crater to its present 40 km diameter would result in large magnitude earthquakes mw 9 3 10 5 and tsunamis in the shallow marine parana karoo basin slope failure and sediment liquefaction are predicted to have occurred within a 700 3000 km radius of the crater causing large scale release of methane from organic rich sediments of this basin including the oil shale horizons of the irati formation new geological evidence for seismicity in the parana basin at the time of impact is presented together with a compilation of existing carbon isotope data from the parana basin which demonstrate a widespread pattern of disturbance consistent with the release of methane these two datasets suggest that both seismicity and methane release took place within ca 1000 km of the impact site with mass balance calculations suggesting ca 1600 gt of methane were released into the atmosphere at this time methane release at this scale would have significant climate effects and would contribute to a sharp

impact of late quaternary environmental changes on deep sea benthic foraminiferal faunas of the red sea abstract fluctuations in abundance diversity and species composition of benthic foraminifera from two sites of the northern and southern red sea indicate strong variability of deep sea ecosystems during the last four glacial to interglacial cycles in total five and four different benthic foraminiferal assemblages have been identified in the northern core and southern core respectively comparison with recent faunas from the red sea and adjacent oceans allowed the reconstruction of temporal changes in deep water ventilation salinity and food availability at the seafloor generally the abundance of infaunal and miliolid taxa increase during glacial intervals indicating increased organic matter fluxes oxygen decrease and salinity increase in deep waters during these times these fluctuations are attributed to enhanced oxygen consumption rates and temporarily reduced deep water formation in the northern red sea during glacial intervals the recorded environmental changes are a reflection of both high and low latitude climate changes the northern red sea is mainly influenced by glacio eustatic sea level fluctuations that control deep water formation rates and by mid latitude climate changes of the mediterranean region that control surface productivity in contrast deep sea ecosystem variability of the southern red sea is additionally influenced by low latitude climate changes attributed to the ne monsoon intensity that drives the inflow of nutrient rich surface waters from the gulf of aden these results demonstrate the high sensitivity of deep sea ecosystems of the red sea to both global and regional climate changes

collapse of large complex impact craters implications from the araguainha impact structure central brazil the 40 km wide araguainha impact structure in central brazil provides extensive outcrops to study the structural evolution of all parts of a complex crater including the central uplift annular trough and crater rim while most craters of comparable size are buried by impact related or postimpact sedimentary deposits araguainha is deeply eroded and it exposes in detail outcrop scale structural features that can be used to understand the structural evolution of large impact craters this study explores evidence from structural features across the entire impact structure in order to provide constraints on the target rock movement during the crater collapse most of the structural features described here are consistent with folding and bedding parallel shearing during several kilometers of lateral inward movement of the target rocks vertical movement was in contrast restricted to distances of less than a few hundred meters along radial and concentric fault zones around the crater rim

palhydaf palaeohydrology in africa program objectives methods major results abstract the palhydaf program aims to reconstruct the late quaternary 130 ka b p to present environmental and hydrological changes of northwest africa and to interpret them in terms of palaeoclimate for testing coupled atmospheric ocean circulation models lacustrine sequences were obtained from closed basins which lie along two meridional transects southern tunisia southern niger and algeria mali particular attention is given to groundwater storage and periods of recharge of the aquifers the chronological framework is based on 14c dates conventional and ams methods and or the th u disequilibrium method in favourable cases e g mali correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge various environmental markers mineralogy elemental chemistry stable isotopes biological remains are used for palaeoenvironmental purposes quantitative estimates of palaeoclimatic variables e g air humidity may be attempted through the combination of geochemical and palaeobiological data the major results already obtained are summarized for the following individual sectors 1 southern tunisia 1 northern algeria 2 southern algeria 3 northern niger 3 mali 4 southern niger at the present stage of investigation the following new inferences can be drown one or several humid episode s occurred in the northern sahara between 150 and 75 ka b p several factors suggest that these humid conditions also affected lower latitudes although their southern extent cannot be given precisely at present the previously postulated existence of large lakes at 20 35 ka b p is not confirmed this time interval seems to coincide with raised water tables but not to a major humid phase in all cases the 14c chronologies obtained for the late pleistocene have to be checked by comparison with other dating methods e g th u disequilibrium after the arid period of the last glacial maximum the return of humid conditions occurred in two major steps at 12 and 9 5 9 ka b p for both the southern and the northern sahara

planar deformation features and impact glass in inclusions from the vredefort granophyre south africa the vredefort granophyre represents impact melt that was injected downward into fractures in the floor of the vredefort impact structure south africa this unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite feldspathic quartzite arkose and granitic material with minor proportions of shale and epidiorite two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features quartz grains in other inclusions display a vermicular texture which is reminiscent of checkerboard feldspar feldspars range from large twinned crystals in some inclusions to fine grained aggregates that apparently are the product of decomposition of larger primary crystals in rare inclusions a mafic mineral probably biotite or amphibole has been transformed to very fine grained aggregates of secondary phases that include small euhedral crystals of fe rich spinel these data indicate that inclusions within the vredefort granophyre were exposed to shock pressures ranging from 5 to 8 30 gpa many of these inclusions contain small rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals including k feldspar quartz augite low ca pyroxene and magnetite the composition of this devitrified glass varies from inclusion to inclusion but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals in the case of granitoid inclusions melt pockets commonly occur at the boundaries between feldspar and quartz grains in metasedimentary inclusions some of these melt pockets contain remnants of partially melted feldspar grains these melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot 650 to 1610 oc impact melt that crystallized to form the vredefort granophyre

petrographic and geochemical characterization of the granitic rocks of the araguainha impact crater brazil petrographic and geochemical data obtained on the araguainha impact crater goias mato grosso states brazil indicate the existence of several molten products that originated during impact induced congruent melting of an alkali granite exposed in the inner part of the central uplift of the structure although previous studies have described these melts to some extent there is no detailed discussion on the petrographic and geochemical variability in the granite and its impactogenic derivatives and therefore little is known about the geochemical behavior and mobility of trace elements during its fusion in the central part of the araguainha crater this paper demonstrates that the preserved granitoid exposed in the core of the structure is a magnesium rich granite similar to postcollisional a type granites also found in terrains outside the araguainha crater in the brasilia orogenic belt the molten products are texturally distinct and different from the original rock but have very similar geochemical composition making it difficult to separate these lithotypes based on concentrations of major and minor elements this also applies for trace and rare earth elements ree thus indicating a high degree of homogenization during impact induced congruent melting under high pressure and postshock temperature conditions petrographic observations along with geochemical data indicate that melting occurs selectively where some of the elements are transported with the melt simultaneously there is an effective dissolution of the rock granite which leads to entrainment of the most resistant solid phases intact or partially molten minerals into the melt minerals more resistant to melting such as quartz and oxides contribute substantially to a chemical balance between the preserved granite and the fusion products generated during the meteoritic impact

post brasiliano pan african high k granitic magmatism in central brazil the role of late precambrian early palaeozoic extension abstract in western goias brazil the emplacement of large high k postorogenic granites and associated small gabbro dioritic intrusions followed immediately after the last deformational events of the brasiliano pan african orogeny at 600 ma well fitted whole rock rb sr isochrons indicate ages which suggest two discrete intrusive events the older between 588 and 560 ma and the younger between 508 and 485 ma the older granites display general petrographic and geochemical characteristics of highly differentiated calc alkaline i type granitoids whereas the younger intrusions are more alkaline similar to a type granites initial 87 sr 86 sr ratios vary from 0 703 to 0 710 and initial nd isotope ratios yield nd t values in the range between 4 0 and 3 0 there are no major differences in initial isotopic compositions between the two granite groups suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the 940 640 ma basement arc type metatonalites metagranodiorites and associated arc metavolcanics the major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust with younger melts being produced by the refusion of anhydrous depleted crustal sources left after the extraction of previous batches of more hydrated calc alkaline magmas the heat input required to promote extensive remelting of the continental crust was most likely provided by mantle derived mafic magmas that invaded and probably underplated the crust during uplift and extension the two intrusive events are bimodal in nature and are interpreted as shallow level extension related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the proterozoic and early palaeozoic the older at 600 ma and the younger between 550 and 510 ma the onset of the granite magmatism at 590 ma shortly post dating the brasiliano orogeny 600 ma is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic paraguay belt in brazil and its correlative in bolivia the tucavaca belt which probably correspond to rift deposits related to the break up of laurentia from gondwana at the end of the proterozoic and beginning of the palaeozoic granites of the younger group cut the deformational structures of the paraguay belt metasediments and pre date by between 40 and 20 ma the initial stages of subsidence and sedimentation of the parana basin

structural evolution of the 40 km wide araguainha impact structure central brazil the 40 km wide araguainha structure in central brazil is a shallowly eroded impact crater that presents unique insights into the final stages of complex crater formation the dominant structural features preserved at araguainha relate directly to the centripetal movement of the target rocks during the collapse of the transient cavity slumping of the transient cavity walls resulted in inward verging inclined folds and a km scale anticline in the outer ring of the structure the folding stage was followed by radial and concentric faulting with downward displacement of kilometer scale blocks around the crater rim the central uplift records evidence for km scale upward movement of crystalline basement rocks from the transient cavity floor and lateral moment of sedimentary target rocks detached from the cavity walls much of the structural grain in the central uplift relates to structural stacking of km scale thrust sheets of sedimentary strata onto the core of crystalline basement rocks outward plunging radial folds indicate tangential oblate shortening of the strata during the imbrication of the thrust sheets each individual sheet records an early stage of folding and thickening due to non coaxial strains shortly before sheet imbrication we attribute this folding and thickening phase to the kilometer scale inward movement of the target strata from the transient cavity walls to the central uplift the outer parts of the central uplift record additional outward movement of the target rocks possibly related to the collapse of the central uplift an inner ring structure at 10 12 km from the crater center marks the extent of the deformation related to the outward movement of the target rocks

cubic zirconia in 2370 c impact melt records earth s hottest crust abstract bolide impacts influence primordial evolution of planetary bodies because they can cause instantaneous melting and vaporization of both crust and impactors temperatures reached by impact generated silicate melts are unknown because meteorite impacts are ephemeral and established mineral and rock thermometers have limited temperature ranges consequently impact melt temperatures in global bombardment models of the early earth and moon are poorly constrained and may not accurately predict the survival stabilization geochemical evolution and cooling of early crustal materials here we show geological evidence for the transformation of zircon to cubic zirconia plus silica in impact melt from the 28 km diameter mistastin lake crater canada which requires super heating in excess of 2370 c this new temperature determination is the highest recorded from any crustal rock our phase heritage approach extends the thermometry range for impact melts by several hundred degrees more closely bridging the gap between nature and theory profusion of 2370 c superheated impact melt during high intensity bombardment of hadean earth likely facilitated consumption of early formed crustal rocks and minerals widespread volatilization of various species including hydrates and formation of dry rigid refractory crust

generation mobilization and crystallization of impact induced alkali rich melts in granitic target rocks evidence from the araguainha impact structure central brazil this paper provides important insights into the generation extraction and crystallization of clast laden impact melt rocks from the araguainha impact structure central brazil despite the mixed nature of the araguainha target rocks comprising a 2 km thick sequence of sedimentary rocks and underlying granitic basement the exposed melt bodies are characterised by an alkali rich granitic matrix embedding mineral and rock fragments derived only from the target granite the melt rocks occur in the form of a massive impact melt sheet overlying the eroded central uplift structure and as melt veins in the granite of the core of the central uplift bulk rock major and trace element data including platinum group elements indicate that the precursor melts were generated locally principally by partial melting of the target granite without any contribution from the sedimentary sequence or the projectile the dense network of melt veins was formed in isolation by selective melting of plagioclase and alkali feldspar within the granite target plagioclase and alkali feldspar melted discretely and congruently producing domains in the matrix of the melt veins which closely match the stoichiometry of these minerals the compositionally discrete initial melt phases migrated through a dense network of microfractures before being assembled into larger melt veins freezing of the melt veins was substantially fast and the melt components were quenched in the form of alkali feldspar and plagioclase schlieren in the matrix of the melt veins the overlying impact melt rock is in contrast characterised by a granophyric matrix consisting of albite sanidine quartz biotite and chlorite in this case melt components appear to have been more mobile and to have mixed completely to form a granitic parental melt we relate the melting of the minerals to post shock temperatures that exceeded the melting point of feldspars

shocked titanite records chicxulub hydrothermal alteration and impact age abstract hydrothermal activity is a common phenomenon in the wake of impact events yet identifying and dating impact hydrothermal systems can be challenging this study provides the first detailed assessment of the effects of shock microstructures and impact related alteration on the u pb systematics and trace elements of titanite catisio5 focusing on shocked granite target rocks from the peak ring of the chicxulub impact structure mexico a 1 mm long shock twinned titanite grain preserves a dense network of irregular microcracks some of which exploit shock twin interfaces secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks in situ laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms analysis reveals a mixture of three end member pb components the pb components are 1 common pb consistent with the pb isotopic signature of adjacent alkali feldspar 2 radiogenic pb accumulated since magmatic crystallization and 3 a secondary younger pb signature due to impact related complete radiogenic pb loss the youngest derived ages define a regression from common pb that intersects concordia at 67 4 ma in agreement with the established age of 66 04 0 05 ma for the chicxulub impact event contour maps of la mc icpms data reveal that the young ages are spatially restricted to microstructurally complex domains that correlate with significant depletion in trace elements ree y zr nb mo sn th and reduction in magnitude of the eu eu anomaly mapping by time of flight secondary ion mass spectrometry tof sims show that patterns of localised element depletion in titanite are spatially related to microcracks which are enriched in al the spatial correlation of ages and trace element abundance is consistent with localised removal of pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid mediated element transport process associated with the impact event here we interpret the 67 4 ma u pb age to represent hydrothermal pb loss in the chicxulub peak ring in the wake of the impact event these results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post impact hydrothermal activity in impact structures elsewhere

insights into the morphology geometry and post impact erosion of the araguainha peak ring structure central brazil the 40 km wide araguainha structure is the largest and best exposed complex impact crater in south america it was excavated in flat lying sediments of the intracratonic parana basin where target rocks are well exposed and have remained undeformed since the impact event ca 245 ma despite the excellent state of preservation and exposure information available on the target rock stratigraphy post impact erosion and morphology of the structure is limited our combined field observations and remote sensing analysis demonstrate that araguainha preserves all features of a shallowly eroded peak ring crater the interior of the structure exposes a central peak surrounded by a 5 km wide annular basin and two main ring features 10 12 and 14 18 km from the center analysis of the pre impact stratigraphy present morphology and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin but minimal to nonexistent beyond the inner ring feature the formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls regional stratigraphic data combined with field observations indicate two periods of post impact erosion associated with exhumation of the parana basin we estimate that 250 350 m of fallback deposits and target rocks have been removed by erosion erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure in contrast to previous suggestions that the impact took place in a shallow marine environment our observations are more consistent with an extremely shallow brackish to freshwater lacustrine environment with little effect on the developing impact crater

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

transformations to granular zircon revealed twinning reidite and zro2 in shocked zircon from meteor crater arizona usa granular zircon in impact environments has long been recognized but remains poorly understood due to lack of experimental data to identify mechanisms involved in its genesis meteor crater in arizona usa contains abundant evidence of shock metamorphism including shocked quartz the high pressure polymorphs coesite and stishovite diaplectic sio2 glass and lechatelierite fused sio2 here we report the presence of granular zircon a new shocked mineral discovery at meteor crater that preserve critical orientation evidence of specific transformations that occurred during formation at extreme impact conditions the zircon grains occur as aggregates of sub micrometer neoblasts in highly shocked coconino sandstone cs comprised of lechatelierite electron backscatter diffraction shows that each grain consists of multiple domains some with boundaries disoriented by 65 around a known 112 shock twin orientation other domains have 001 in alignment with 110 of neighboring domains consistent with the former presence of the high pressure zrsio4 polymorph reidite additionally nearly all zircon preserve zro2 sio2 providing evidence of partial dissociation the genesis of cs granular zircon started with detrital zircon that experienced shock twinning and reidite formation at pressures from 20 to 30 gpa ultimately yielding a phase that retained crystallographic memory this phase subsequently recrystallized to systematically oriented zircon neoblasts and in some areas partially dissociated to zro2 the lechatelierite matrix experimentally constrained to form at 2000 c provided the ultrahigh temperature environment for zircon dissociation 1670 c and neoblast formation the capacity of granular zircon to preserve a cumulative pressure temperature record has not been recognized previously and provides a new method for investigating histories of impact related mineral transformations in the crust at conditions far beyond those at which most rocks melt

mesoscale eddies in the gulf of aden and their impact on the spreading of red sea outflow water abstract the gulf of aden goa in the northwestern indian ocean is the receiving basin for red sea outflow water rsow one of the world s few high salinity dense overflows but relatively little is known about spreading pathways and transformation of rsow through the gulf here we combine historical data satellite altimetry new synoptic hydrographic surveys and the first in situ direct observations of subsurface currents in the goa to identify the most important processes in the spreading of rsow the new in situ data sets were collected in 2001 2003 as part of the red sea outflow experiment redsox and consist of two ctd ladcp surveys and 49 one year trajectories from acoustically tracked floats released at the depth of rsow the results indicate that the prominent positive and negative sea level anomalies frequently observed in the goa with satellite altimetry are associated with anticyclonic and cyclonic eddies that often reach to at least 1000 m depth i e through the depth range of equilibrated rsow the eddies dominate rsow spreading pathways and help to rapidly mix the outflow water with the background eddies in the central and eastern gulf are basin scale 250 km diameter and have maximum azimuthal speeds of about 30 cm s at the rsow level in the western gulf smaller eddies not detectable with satellite altimetry appear to form as the larger westward propagating eddies impale themselves on the high ridges flanking the tadjura rift both the hydrographic and lagrangian observations show that eddies originating outside the gulf often transport a core of much cooler fresher water from the arabian sea all the way to the western end of the goa where the highest salinity outflow water is found this generates large vertical and horizontal gradients of temperature and salinity setting up favorable conditions for salt fingering and diffusive convection both of these mixing processes were observed to be active in the gulf two new annually appearing anticyclonic eddies are added to the previously identified gulf of aden eddy gae prasad and ikeda 2001 and somali current ring scr fratantoni et al 2006 these are the summer eddy se and the lee eddy le both of which form at the beginning of the summer monsoon when strong southwest winds blowing through socotra passage effectively split the gae into two smaller eddies the se strengthens as it propagates westward deeper in the goa while the lee eddy remains stationary in the lee of socotra island both eddies are strengthened or sustained by ekman convergence associated with negative wind stress curl patches caused by wind jets through or around high orography the annual cycle in the appearance propagation and demise of these new eddies and those described in earlier work is documented to provide a comprehensive view of the most energetic circulation features in the goa the observations contain little evidence of features that have been shown previously to be important in the spreading of mediterranean outflow water mow in the north atlantic namely a wall bounded subsurface jet the mediterranean undercurrent and submesoscale coherent lenses containing a core of mow meddies this is attributed to the fact that the rsow enters the open ocean on a western boundary high background eddy kinetic energy typical of western boundary regimes will tend to shear apart submesoscale eddies and boundary undercurrents even if a submesoscale lens of rsow did form in the goa westward self propagation would transport the eddy and its cargo of outflow water back toward rather than away from its source

u pb geochronology of apatite and zircon from the brent impact structure canada a late ordovician sandbian katian boundary event associated with l chondrite parent body disruption in situ la icp ms u pb geochronology has been performed on apatite and zircon within thermally recrystallized clast laden and clast poor impact melt rocks from the brent impact structure a total of 377 laser analyses on 120 impact melt grown n 9 and impact recrystallized zircon grains n 111 were obtained from which a concordia age of 452 8 2 7 ma mswd 0 57 n 11 and a weighted average mean 206pb 238u age of 453 2 2 9 ma mswd 0 60 n 11 are calculated a total of 300 laser analyses from 100 relict apatite grains were obtained with an unanchored regression through all data yielding a lower intercept age of 453 2 6 0 ma mswd 5 8 n 300 that overlaps within error of zircon 207pb 206pb ratios obtained from feldspar clasts within clast laden impact melt retain the same initial pb composition as the target rocks from which they are derived while feldspars that crystallized from impact melt have 207pb 206pb ratios indicative of isotopic re equilibration between basement lithologies of two different ages a similar variability in 207pb 206pb is recorded by apatite this provides evidence for the involvement of neoproterozoic lake nipissing alkaline suite as well as mesoproterozoic grenville gneisses in the production of impact melt at brent recrystallized apatite grains exhibit enrichments in light rare earth elements lrees along neoblast grain boundaries indicative of trace element substitution and phase precipitation during impact induced recrystallization an age of 452 8 2 7 ma from zircon and 453 2 6 0 ma from apatite places the impact event in the late ordovician at or near the sandbian katian boundary confirming brent s involvement in the middle to late ordovician crater cluster event a period of enhanced impactor flux to earth related to the l chondrite parent body disruption

linking shock textures revealed by bse cl and ebsd with u pb data la icp ms and sims from zircon from the araguainha impact structure brazil

nanoscale records of ancient shock deformation reidite zrsio4 in sandstone at the ordovician rock elm impact crater the terrestrial record of meteorite impacts is difficult to decipher because unequivocal evidence of impact is increasingly destroyed with time by erosion burial and tectonics zircon survives these processes as a shocked mineral and above 20 gpa transforms to reidite a high pressure zrsio 4 polymorph diagnostic of impact however the utility of reidite has been limited by its occurrence it has only been reported from three relatively young 36 ma impact craters globally here we report a new occurrence of reidite in brecciated sandstone from the ordovician rock elm impact crater in wisconsin united states electron backscatter diffraction mapping was used to identify reidite and microtwins within shocked zircons smaller than 50 m in diameter reidite occurs both as 200 500 nm wide lamellar intergrowths and as nanoparticulate grains and not only provides the first diagnostic evidence for ultrahigh pressure shock metamorphism at rock elm but is also the oldest reported occurrence of reidite considering its small size and the ubiquitous presence of detrital zircon in siliciclastic rocks reidite may be more common in the rock record than has been reported but has potentially gone undetected the recognition that nanoscale reidite can be preserved over deep time within zircon in shock metamorphosed sandstone presents new opportunities for investigating earth9s impact record as it could potentially preserve nanoscopic evidence of impact events much older than the one that formed rock elm given that shocked zircons have been shown to survive sedimentary cycling the identification of reidite within zircons in siliciclastic rocks could facilitate investigating the impact chronology over much of the geological time scale as the oldest terrestrial minerals known are detrital zircons

spatial u pb age distribution in shock recrystallized zircon a case study from the rochechouart impact structure france abstract age determination of impact structures via the zircon u pb system remains challenging and often ambiguous due to highly variable effects of shock metamorphism on u pb geochronology it is therefore crucial to link the observed zircon microtextures including their temperature and pressure conditions associated with their formation directly to the u pb ages preserved here we analyzed three recrystallized zircon grains and one plastically deformed zircon crystal from the medium sized rochechouart impact structure in the northwestern massif central of france for the rochechouart impact structure the impact age 206 92 0 32 ma 40ar 39ar as well as the tectono themal history is well established making this study site ideal to test concepts about u pb systematics in shocked zircon and to differentiate between shock driven age resetting and pre impact crystallization and metamorphic overprinting zircon microstructures were studied using scanning electron imaging cathodoluminescence imaging and electron backscatter diffraction ebsd mapping further we conducted u pb laser ablation inductively coupled plasma mass spectrometry la icp ms depth profiling analysis allowing us to interpret the resultant age data in discrete steps with increasing ablation time depth the u pb depth profiling data demonstrate that plastically strained grains are incompletely reset and preserve rim and interior age domains reflecting typical pre impact pre variscan and variscan regional tectonic ages our results also reveal that the granular crystals encountered contain microstructural evidence for former reidite in granular neoblastic frign zircon reflecting both high pressure 30 gpa and high temperature 1200 c conditions this signifies that frign zircon is now known from an additional medium sized impact structure further supporting the hypothesis that this impact induced microstructure is commonly preserved in addition frign zircon has a high potential to preserve completely impact reset crystal domains 204 to 207 ma that can be identified by our combined analytical approach of u pb depth profiling and ebsd mapping and thus are suitable for determining reliable impact ages

on the significance of crater ages new ages for dellen sweden and araguainha brazil samples of a few grams in weight from the impact melt at the dellen crater sweden and the araguainha dome brazil were analyzed by rb sr and sm nd techniques for dellen pure labradorite orthopyroxene and glass separates from a clast free impact melt rock with holohyaline ground mass yield an internal rb sr isochron with 89 0 2 7 ma isr 0 83571 0 00007 2 compared to ar ar plateau ages of 102 3 2 ma and 109 6 2 0 ma published for total melt rocks various whole rock values not plotting on this rb sr isochron indicate primary variations in 87sr86sr over the melt sheet the age of the araguainha dome brazil is bracketed by the rb sr model age of 243 19 ma isr 0 7226 0 0005 for altered cordierite in the impact melt and by the internal rb sr isochron age of 449 9 ma isr 0 7140 0 0007 for an uplifted granite shock stage ib in melt rocks from both impact structures 143nd144nd was not homogenized during total rock melting sm nd data either scatter around the primary isochron araguainha or point to a partial resetting dellen this data array may be caused by tiny refractory ree rich inclusions in the main mineral phases which dominate the sm nd system of the impact melt rocks the significant difference between the new isochron age for dellen and ar ar ages show that the data base for analysis of periodicity in cratering record most probably lack precision and accuracy

shocked monazite chronometry integrating microstructural and in situ isotopic age data for determining precise impact ages monazite is a robust geochronometer and occurs in a wide range of rock types monazite also records shock deformation from meteorite impact but the effects of impact related microstructures on the u th pb systematics remain poorly constrained we have therefore analyzed shock deformed monazite grains from the central uplift of the vredefort impact structure south africa and impact melt from the araguainha impact structure brazil using electron backscatter diffraction electron microprobe elemental mapping and secondary ion mass spectrometry sims crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal plastic deformation features including shock twins planar deformation bands and neoblasts shock twins were documented in up to four different orientations within individual monazite grains occurring as compound and or type one twins in 001 100 left 10 bar 1 right 110 left 212 right and type two irrational twin planes with rational shear directions in 0 bar 1 bar 1 and bar 1 bar 1 0 sims u th pb analyses of the plastically deformed parent domains reveal discordant age arrays where discordance scales with increasing plastic strain the correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event neoblasts in granular monazite domains are strain free having grown during the impact events via consumption of strained parent grains neoblastic monazite from the inlandsee leucogranofels at vredefort records a 207pb 206pb age of 2010 15 ma 2 n 9 consistent with previous impact age estimates of 2020 ma neoblastic monazite from araguainha impact melt yield a concordia age of 259 5 ma 2 n 7 which is consistent with previous impact age estimates of 255 3 ma our results demonstrate that targeting discrete microstructural domains in shocked monazite as identified through orientation mapping for in situ u th pb analysis can date impact related deformation monazite is therefore one of the few high temperature geochronometers that can be used for accurate and precise dating of meteorite impacts

deep sea benthic foraminiferal assemblage response to a surface ocean productivity gradient a test a transect of samples from the eastern equatorial pacific where surface ocean productivity is the only changing environmental variable was used to test the response of benthic foraminiferal assemblages to an open ocean productivity gradient the transect runs along the east pacific rise from the highly productive waters near the equator to the oligotrophic south pacific subtropical gyre surface sediment benthic assemblages show progressive change along the transect with strong compositional shifts corresponding to the rapid productivity gradient near the equator and to the position of the southern convergence near 18 19 s the relationship of the assemblages to surface ocean productivity was tested by statistical regression of quantitative estimates of productivity on the assemblage principal component scores calculated for each sampling location the regression yielded an r squared of 0 87 demonstrating a clear relationship between surface ocean productivity and deep sea benthic foraminiferal assemblage composition the species most important to this relationship display a consistent response to oceanic productivity for long stretches of geologic time this conservative behavior and the results of the regression analysis indicate that benthic foraminiferal assemblages can be used to estimate surface ocean productivity for the geologic past

liquefaction of sedimentary rocks during impact crater development abstract impact crater development on every planetary body requires catastrophic movement of large volumes of crustal rocks the process produces well known features such as brecciation and frictional melting but a mechanism that explains how rocks accommodate the strain during the cratering flow remains unclear here we investigate target rocks from the araguainha impact crater central brazil that typify what happens to a consolidated fluid saturated sedimentary rock at 2 km below the surface prior to the impact event sandstone units record a pattern of chaotic large scale folds and pervasive microscopic grain to grain brecciation that result from rock strength degradation triggered by the impact field mapping and extensive textural observations indicate that these sandstones experienced initial microstructural damage from the shock wave and that this process may have weakened grain to grain bonds and started the process of pervasive microbrecciation accompanying heating and decompression lead to vaporization and expansion of fluids in the sandstone pores magnifying the process of brecciation by effectively liquefying the rock mass and allowing for chaotic folding at a range of scales up to blocks 100 m in length in the central uplift this is a vaporization assisted microbrecciation and it may have inhibited the formation of pseudotachylites because energy was dissipated by pervasive microcracking vaporization of pore fluids and large scale chaotic folding rather than localized displacement on brittle faults and frictional heating we suggest that impact liquefaction of sedimentary rocks depends on whether the presence of pore fluids and related micro brecciation are sufficient to dissipate most of the impact energy

response of cathodoluminescence to crystal plastic deformation in zircon geochemical and geochronological studies of zircon are commonly supplemented by cathodoluminescence cl imaging because it provides a means of recognizing different generations of zircon growth at high spatial resolution crystal plastic deformation of zircon can have significant effects on zircon geochemistry detailed analyses from electron backscatter diffraction mapping combined with panchromatic cl imaging and hyperspectral cl mapping of several crystal plastically deformed grains from different geological settings are used to establish the relationships between crystal plastic deformation and cl in zircon results show a strong spatial association between deformation microstructures and cl response that lead to modification of cl that commonly cross cuts primary zoning variable contributions from two fundamental deformation related processes result in a variety of cl characteristics a defect control on panchromatic cl intensity particularly at low angle subgrain boundaries and changes in spectral cl response due to deformation related modification of cl active ree geochemistry a framework is provided for the recognition of deformation related microstructures using cl and the usefulness of cl imaging in the discrimination of these microstructures is critically evaluated

fingerprinting the k t impact site and determining the time of impact by upb dating of single shocked zircons from distal ejecta abstract u pb isotopic dating of single 1 3 g zircons from k t distal ejecta from a site in the raton basin colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site data for the least shocked zircons are slightly displaced from the 544 5 ma primary age for a component of the target site white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65 5 3 0 ma such shocked and granular zircons have never been reported from any source including explosive volcanic rocks zircon is refractory and has one of the highest thermal blocking temperatures hence it can record both shock features and primary and secondary ages without modification by post crystallization processes unlike shocked quartz which can come from almost anywhere on the earth s crust shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers with u pb zircon dating it is now possible to correlate ejecta layers derived from the same target site test the single versus multiple impact hypothesis and identify the target source of impact ejecta the ages obtained in this study indicate that the manson impact site iowa which has basement rocks that are mid proterozoic in age cannot be the source of k t distal ejecta the k t distal ejecta probably originated from a single impact site because most grains have the same primary age

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

microstructural constraints on the mechanisms of the transformation to reidite in naturally shocked zircon zircon zrsio4 is used to study impact structures because it responds to shock loading and unloading in unique crystallographically controlled manners one such phenomenon is the transformation of zircon to the high pressure polymorph reidite this study quantifies the geometric and crystallographic orientation relationships between these two phases using naturally shocked zircon grains reidite has been characterized in 32 shocked zircon grains shocked to stages ii and iii using a combination of electron backscatter diffraction ebsd and focused ion beam cross sectional imaging techniques the zircon bearing clasts were obtained from within suevite breccia from the nordlingen 1973 borehole close to the center of the 14 4 ma ries impact crater in bavaria germany we have determined that multiple sets up to 4 of reidite lamellae can form in a variety of non rational habit planes within the parent zircon however ebsd mapping demonstrates that all occurrences of lamellar reidite have a consistent interphase misorientation relationship with the host zircon that is characterized by an approximate alignment of a 100 zircon with a 112 reidite and alignment of a 112 zircon with a conjugate 112 reidite given the tetragonal symmetry of zircon and reidite we predict that there are eight possible variants of this interphase relationship for reidite transformation within a single zircon grain furthermore laser raman mapping of one reidite bearing grain shows that moderate metamictization can inhibit reidite formation thereby highlighting that the transformation is controlled by zircon crystallinity in addition to lamellar reidite submicrometer scale granules of reidite were observed in one zircon the majority of reidite granules have a topotaxial alignment that is similar to the lamellar reidite with some additional orientation dispersion we confirm that lamellar reidite likely forms via a deviatoric transformation mechanism in highly crystalline zircon whereas granular reidite forms via a reconstructive transformation from low crystallinity zrsio4 within the reidite stability field the results of this study further refine the formation mechanisms and conditions of reidite transformation in naturally shocked zircon

coherent response of arabian sea upwelling and pollen transport to late quaternary monsoonal winds the indian summer monsoon causes large seasonal changes in the environments of both eastern africa and the arabian sea here we compare time series 140 kyr long of selected pollen types and foraminiferal upwelling faunas preserved in marine sediments to identify the frequencies of their variability and the coherence and phase of the terrestrial and marine responses to monsoonal circulation during interglacial intervals the pollen and upwelling records of the western arabian sea indicate stronger monsoons and are both coherent and in phase at periodicities near the precession of the earth s rotational axis 23 kyr we conclude that the strength of the monsoonal southwesterlies accounts for this high coherence of marine and terrestrial records during interglacial intervals low correlation during glacial phases indicates weaker monsoons and some decoupling of these two records these data indicate that two distinct modes of monsoon variability may exist an interglacial mode with strong monsoons and effective coupling of terrestrial and marine records and a glacial mode with both weaker monsoons and decreased linkage between arabian sea pollen and upwelling records

benthic foraminiferal successions across late quaternary mediterranean sapropels abstract benthic foraminiferal records across late quaternary mediterranean sapropels s6 s5 and s1 are reviewed and re considered in the light of recent advances in foraminiferal ecology it is suggested that the main factor controlling the foraminiferal successions that occur immediately before and after sapropel deposition is the amount of time involved in the onset of anoxic conditions and in the re oxygenation of the benthic environment faunas dominated by deep infaunal taxa such as globobulimina and chilostomella reflect a very gradual decrease or increase of bottom water oxygenation post sapropel faunas dominated by small biconvex trochospiral taxa which are inferred to have a much more opportunistic life strategy are typical of a very rapid re oxygenation of the benthic environment after sapropel deposition the time involved in re oxygenation depends upon the mechanism causing sapropel formation the faunal succession found above sapropel s5 suggests a basin with a strong density gradient in the upper part of the water column during sapropel deposition followed by a rapid turnover of the following column and quick re oxygenation of the benthic environment in contrast the faunal succession above sapropel s6 suggests a water column with a more gradual density gradient deeper in the basin a progressive deepening of the halocline and a very slow increase of bottom water oxygenation

benthic foraminifera as proxies of organic matter flux and bottom water oxygenation a case history from the northern arabian sea abstract benthic foraminifera have been quantified in two sediment cores from a topographic high murray ridge in the northern arabian sea one core is from a station within the present day oxygen minimum zone omz the other from a station below the omz changes in the intensity of the omz linked to monsoonal climate variability had a strong impact on benthic foraminiferal composition and accumulation rates at the shallower omz station 920 m an intensification of the omz resulted in lower diversities and high dominances of low oxygen tolerant species the deeper station 1470 m was far less affected though at least once during the last 225 000 years the benthic faunal composition and accumulation rates show evidence for a deepening of the base of the omz to a level close to or even below 1470 m we suggest that percentages of miliolids can be used for rapid reconstruction of periods of increased ventilation in the northern arabian sea and are a promising proxy for tracing changes in bottom and pore water oxygenation in general omz conditions are characterised by high abundances of deep infaunal species and increased burial efficiency of organic matter the proportion of deep infaunal species appears to be a measure for the anoxic burial efficiency of organic matter and not a direct measure for the arriving flux at the seabed results furthermore show that benthic foraminiferal accumulation rates cannot be used as a proxy for surface water productivity under the prevalence of severe dysoxia