the sedimentary expression of oceanic anoxic event 1b in the north atlantic abstract the aptian albian oceanic anoxic event 1b contains the record of severalperturbations in the global carbon cycle and multiple black shale levels particularly in the western tethys the local lithological expression of anoceanic anoxic event depends on palaeogeographical and depositional settingas well as on regional palaeoclimate marine sediments at a particular locationmay therefore be more or less organic rich or not at all and they may consistof different lithologies in most studies however much of the lithologicalvariability associated with oceanic anoxic events is left unaccounted for and thus the exact processes leading to the enrichment of organic matter in thesemarine sediments and their subsequent preservation in the geological recordare unknown this study focuses on the local sedimentary processes behindthe deposition of organic rich sediments at deep sea drilling project site 545and ocean drilling program sites 1049 and 1276 in the north atlantic duringoceanic anoxic event 1b although speci cally dealing with the sedimentsdeposited during this particular event at these localities it is expected that thesame processes were responsible for determining the exact sedimentaryproducts at localities in similar settings as well as during other similar eventsin the mesozoic here it is shown that the deposition of organic richsediments during oceanic anoxic event 1b was a consequence of theenhanced productivity favoured by upwelling and by riverine nutrientinput or aeolian fertilization of the euphotic zone depending ongeographical location slope instability processes resulted in the transfer ofpart of these organic rich sediments from the shelf to deep sea depocentres asmud laden organic rich turbidites especially in the northern north atlantic the so called black shales are much more varied than their name implies the end product of sedimentation during an oceanic anoxic event at aparticular location is commonly the result of several equi nal processes actingon a local scale rather than the direct result of basinal or even globalmechanisms keywords aptian albian kilian event north atlantic oae 1b oceandrilling program oceanic anoxic event paquier event

an 15 000 year record of el nino driven alluviation in southwestern ecuador debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the pacific ocean in ecuador these storm induced events were dated by radiocarbon and the age of laminae that are less than 200 years old matches the historic record of el nino events from about 15 000 to about 7000 calendar years before the present the periodicity of clastic deposition is greater than or equal to 15 years thereafter there is a progressive increase in frequency to periodicities of 2 to 8 5 years this is the modern el nino periodicity which was established about 5000 calendar years before the present this may reflect the onset of a steeper zonal sea surface temperature gradient which was driven by enhanced trade winds

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

holocene palaeoclimate records over europe and the north atlantic this special issue of the holocene contains 16 research papers based on a symposium at the 11th international meeting of the european union of geosciences held in strasbourg in april 2001 the aim of the symposium was a state of the art assessment of empirical studies of postglacial marine and terrestrial climatic archives and their integration with numerical climate models this editorial places the individual papers in the broader context of natural climate variability and anthropogenic impacts on the global climate system regional differences in climate between maritime and continental areas and the need for an improved theoretical basis for understanding the underlying causes of environmental change the focus of the special issue is the dynamic and relatively well understood climate of the north atlantic and the european realm where in relation to the steepest offshore temperature gradient on earth observational data are abundant and many recent advances have been made in climate reconstruction

australasian evidence for mid holocene climate change implies precessional control of walker circulation in the pacific abstract at its simplest the pacific cell of the walker circulation may be regarded as a thermal circulation driven by the temperature contrast between the west and east pacific evidence from australasia is summarized indicating that this circulation was greatly weakened in the early holocene but was enhanced at about 5000 bp the initial effect of this enhancement was the intensification of the northern australian monsoon within the australian tropics but this was reversed abruptly after 3700 bp the enhancement is attributed to an increased polar equator pressure gradient after 5000 bp with consequent intensification of the overall circulation pattern in the southern hemisphere including the mid latitude westerlies and the trade winds this in turn appears to be a result of increased southern hemisphere seasonality driven by the precessional cycle it is proposed that increased wind speeds off south america after 5000 bp in both mid and low latitudes enhanced upwelling and reduced sea surface temperatures ssts this created a greater east west surface temperature contrast across the pacific compounding a pre existing trend towards enhanced circulation and flipping climates in the southern hemisphere pacific basin from an early holocene mode to a late holocene mode under reduced circulation conditions early holocene climates at low latitudes in the southern hemisphere may have been dominated by monsoonal heat transfers

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

el nino variability off peru during the last 20 000 years here we present a high resolution marine sediment record from the el nino region off the coast of peru spanning the last 20 000 years sea surface temperature photosynthetic pigments and a lithic proxy for el nino flood events on the continent are used as paleo el nino southern oscillation proxy data the onset of stronger el nino activity in peru started around 17 000 calibrated years before the present which is later than modeling experiments show but contemporaneous with the heinrich event 1 maximum el nino activity occurred during the early and late holocene especially during the second and third millennium b p the recurrence period of very strong el nino events is 60 80 years el nino events were weak before and during the beginning of the younger dryas during the middle of the holocene and during medieval times the strength of el nino flood events during the last millennium has positive and negative relationships to global and northern hemisphere temperature reconstructions

340 000 year centennial scale marine record of southern hemisphere climatic oscillation in order to investigate rapid climatic changes at mid southern latitudes we have developed centennial scale paleoceanographic records from the southwest pacific that enable detailed comparison with antarctic ice core records these records suggest close coupling of mid southern latitudes with antarctic climate during deglacial and interglacial periods glacial sections display higher variability than is seen in antarctic ice cores which implies climatic decoupling between mid and high southern latitudes due to enhanced circum antarctic circulation structural and temporal similarity with the greenland ice core record is evident in glacial sections and suggests a degree of interhemispheric synchroneity not predicted from bipolar ice core correlations

holocene history of the el nino phenomenon as recorded in flood sediments of northern coastal peru significant precipitation along the north central coast of peru lat 5 10 s occurs exclusively during el nino incursions of warm water into the peruvian littoral flood deposits from this region therefore provide a proxy record of extreme el nino events i present a 3500 yr chronology of the extreme events based on radiocarbon dating of overbank flood sediments from the rio casma lat 9 2 s the flood plain stratigraphy suggests that the el nino phenomenon has occurred throughout the holocene and that flood events much larger than that which occurred during 1982 1983 occur here at least once every 1000 yr

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

mineralogical and spectroscopic investigation of enstatite chondrites by x ray diffraction and infrared reflectance spectroscopy 1 the mineralogy and infrared reflectance spectra of 13 enstatite e chondrite meteorite finds spanning the full range of textural alteration grades in both el and eh classes have been investigated rietveld refinement of high resolution powder x ray diffraction xrd data was used to determine quantitative major mineral abundances sample correlated mid infrared 2 0 to 25 0 m 4500 cm 1 to 400 cm 1 reflectance infrared spectra were collected for each meteorite spectral features due to the fundamental lattice vibrations of the silicates primarily enstatite dominate the spectra of these meteorites over most of the spectral range investigated the spectral features related to primary i e pre terrestrial mineralogy include fundamental stretching and bending lattice modes 8 3 25 0 m 1200 400 cm 1 overtones and combinations of the fundamental modes 4 5 6 1 m 2200 1650 cm 1 and the principle christensen feature 8 3 m 1200 cm 1 terrestrial weathering products including fe oxyhydroxides gypsum and carbonates occur in most of these meteorites and contribute to some spectral features particularly an asymmetric feature near 2 6 to 3 8 m 3800 to 2600 cm 1 attributed to adsorbed hydrogen bonded and or structural oh and h 2 o and a feature near 6 2 m 1625 cm 1 attributed to adsorbed hydrogen bonded and or structural h 2 o modal mineral abundances determined by rietveld refinement have been used to calculate model grain densities for each meteorite bulk magnetic susceptibility measurements combined with modal mineralogy and grain densities reveal a trend toward lower grain density and lower bulk susceptibility with increased terrestrial weathering

the portales valley meteorite breccia evidence for impact induced melting and metamorphism of an ordinary chondrite abstract the portales valley h chondrite fall is an annealed impact melt breccia with coarse metal interstitial to angular and subrounded silicate clasts the large metal rich regions exhibit a widmanstatten structure and contain very little troilite we were able to examine a 16 5 kg metal rich specimen of portales valley silicates contain numerous flecks of metallic cu and curvilinear trails of tiny metallic fe ni blebs characteristic of shocked and annealed chondrites one silicate clast appears to have experienced little

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

sedimentation of lithogenic particles in the deep ocean abstract investigation of lithogenic particles collected by sediment traps in open ocean stations revealed that the sediment flux increased linearly with depth in the water column this rate of increase decreased with distance of the station from the continent it was largest at the panama basin station and almost negligible at the e hawaii abyssal plain station at the panama basin station smectite flux increased with depth we suggest that smectite resuspended from bottom sediments of the continental slope west of the sediment trap station is advected by easterly deep currents and the suspended particles are then possibly entrapped by large settling particles on the other hand the flux of hemipelagic clay particles kaolinite and chlorite was nearly constant at all depths this can be explained by incorporation of these particles in fecal pellets which then settle from the surface water at the demerara abyssal basin station flux of illite and chlorite particles increased with depth and the flux of smectite was constant a sudden increase of the flux of illite and chlorite was observed near the bottom traps at the sohm abyssal plain station the flux of quartz and feldspar was 10 to 15 of the clay flux

shock features in iron nickel metal and troilite of l group ordinary chondrites shock metamorphic features in opaque minerals feni metal and troilite of 22 l chondrites have been studied petrographically and geochemically in an attempt to establish a connection between the present silicate based shock classification scheme stoffler et al 1991 and the peak shock and postshock thermal history recorded in these minerals unshocked to weakly shocked s1 s3 l chondrites contain feni metal and troilite that display textures related to normal slow cooling they may also contain rare disequilibrium shock features which suggest localized departures from equilibrium shock conditions above shock stage s3 selected melting of feni metal and troilite produces melt droplets whose composition and abundance correspond to the maximum equilibrium shock state achieved by the sample at these higher shock levels the abundance of other shock induced features such as polycrystalline kamacite sheared and fizzed troilite coarse grained pearlitic plessite polycrystalline troilite and polymineralic melt veins serve as textural criteria that can be used to establish peak shock conditions minimum postshock temperatures obtained from analyses of plessite components show a systematic increase in temperature with an increase in shock stage thereby providing additional information about the postshock thermal histories of l chondrites at the highest shock levels recorded in l chondrites s6 and above melting and chemical homogenization of feni metal produces flattened ni profiles that may partially to completely obscure any evidence for an earlier slow cooling history all of these features serve as aids for shock classifying l chondrites as well as for quantifying minimum peak temperatures that resulted during shock metamorphism

quasi quadrennial and quasi biennial variability in the equatorial pacific evaluation of competing el nino southern oscillation enso theories requires one to identify separate spectral peaks in equatorial wind and sea surface temperature sst time series to sharpen this identification we examine the seasonal to interannual variability of these fields by the data adaptive method of multi channel singular spectrum analysis m ssa m ssa is applied to the equatorial band 4 n 4 s using 1950 1990 data from the comprehensive ocean and atmosphere data set two major interannual oscillations are found in the equatorial sst and surface zonal wind fields u the main peak is centered at about 52 months we refer to it as the quasi quadrennial qq mode quasi biennial qb variability is split between two modes with periods near 28 months and 24 months a faster 15 month oscillation has smaller amplitude the qq mode dominates the variance and has the most distinct spectral peak in time longitude reconstructions of this mode the sst has the form of a standing oscillation in the eastern equatorial pacific while the u field is dominated by a standing oscillation pattern in the western pacific and exhibits also slight eastward propagation in the central and western pacific the locations of maximum anomalies in both qb modes are similar to those of the qq mode slight westward migration in sst across the eastern and central and eastward propagation of u across the western and central pacific are found the significant wind anomaly covers a smaller region than for the qq the qq and qb modes together represent the enso variability well and interfere constructively during major events the sharper definition of the qq spectral peak and its dominance are consistent with the devil s staircase interaction mechanism between the annual cycle and enso

high resolution temperature and evaporation changes during the late holocene in the northeastern arabian sea in order to reconstruct the monsoonal variability during the late holocene we investigated a complete annually laminated sediment record from the oxygen minimum zone omz off pakistan for oxygen isotopes of planktic foraminifera and alkenone derived sea surface temperatures sst significant sst changes of up to 3 c which cannot be explained by changes in the alkenone producing coccolithophorid species inferred from the gephyrocapsa oceanica emiliania huxleyi ratio suggest that sst changes are driven by changes in the monsoon strength our high decadal resolution data indicate that the late holocene in the northeastern arabian sea was not characterized by a stable uniform climate as inferred from the greenland ice cores but by variations in the dominance of the sw monsoon conditions with significant effects on temperatures highest sst fluctuations of up to 3 0 c and 2 5 c were observed for the time interval from 4600 to 3300 years b p and during the past 500 years the significant short term sst changes during the past 500 years might be related to climatic instabilities known from the northern latitudes little ice age and confirm global effects surface salinity values reconstructed from 18o records after correction for temperature related oxygen isotope fractionation suggest that in general the past 5000 years were characterized by higher than recent evaporation and more intense sw monsoon conditions however between 4600 and 3700 years b p evaporation dropped sw monsoon weakened and ne monsoon conditions were comparatively enhanced for the past 1500 years we infer strongly fluctuating monsoon conditions comparisons of reconstructed salinity records with ice accumulation data from published tibetan ice core and tibetan tree ring width data reveal that during the past 2000 years enhanced evaporation in the northeastern arabian sea correlates with periods of increased ice accumulation in tibet and vice versa this suggests a strong climatic relationship between both monsoon controlled areas

a euxinic southern north atlantic ocean during the cenomanian turonian oceanic anoxic event abstract during mid cretaceous times large amounts of organic carbon corg became sequestered in black shales possibly due to oceanic anoxic events characterized by the development of an extended oxygen minimum zone omz here we provide the first direct evidence for an open ocean omz in the cenomanian turonian c t southern north atlantic and in fact show that the base of the photic zone was euxinic as revealed by molecular fossils from photosynthetic green sulfur bacteria in c t black shales this together with evidence for bottom water anoxia and accumulation of redox sensitive trace metals and hydrogen rich organic matter indicates a continuously euxinic water column concurrent with the high corg accumulation rates which are 15 150 times greater in the southern than in the northern north atlantic and the low biological productivity this suggests that preservation controlled the accumulation of corg in c t black shales

carotenoid diagenesis in recent marine sediments i the peru continental shelf 15 s 75 w abstract we report here the distribution of carotenoids in peru marine surface sediments including partial identification of 37 pigments 33 of which have not been previously reported to occur in marine sediments our analysis demonstrates that zooplanktonic crustacea diatoms dinoflagellates cryptomonads purple nonsulfur photosynthetic bacteria and unidentified species of photosynthetic bacteria are contributors to sedimentary organic matter fucoxanthin fucoxanthinol diadinoxanthin peridinin and peridininol the principal carotenoids of diatoms and dinoflagellates observed in sediment traps are nearly absent from our sample even though these phytoplankton are the primary source of organic matter we attribute this to the presence of 5 6 epoxides in these pigments a feature which causes rapid cleavage of the polyene and fragmentation to low molecular weight compounds

enso like forcing on oceanic primary production during the late pleistocene late pleistocene changes in oceanic primary productivity along the equator in the indian and pacific oceans are revealed by quantitative changes in nanoplankton communities preserved in nine deep sea cores we show that variations in equatorial productivity are primarily caused by glacial interglacial variability and by precession controlled changes in the east west thermocline slope of the indo pacific the precession controlled variations in productivity are linked to processes similar to the southern oscillation phenomenon and they precede changes in the oxygen isotopic ratio which indicates that they are not the result of ice sheet fluctuations the 30 000 year spectral peak in the tropical indo pacific ocean productivity records is also present in the antarctica atmospheric co2 record suggesting an important role for equatorial biological productivity in modifying atmospheric co2

last interglacial and early glacial enso although the link between insolation and climate is commonly thought to be in the high northern latitudes in summer our results show that the start of the last glaciation in marine isotope stage mis 5d was associated with a change of insolation during the transitional seasons in the low latitudes a simplified coupled ocean atmosphere model shows that changes in the seasonal cycle of insolation could have altered el nino southern oscillation enso variability so that there were almost twice as many warm enso events in the early glacial than in the last interglacial this indicates that ice buildup in the cooled high latitudes could have been accelerated by a warmed tropical pacific

glacial interglacial dynamics of the eastern equatorial pacific cold tongue intertropical convergence zone system reconstructed from oxygen isotope records record mean late holocene lh last glacial maximum lgm d 18 o amplitudes ranging between 1 0 and 1 3 we estimate that mean sea surface temperatures sst in this region during the lgm were on average 1 5 0 5 c lower than the lh larger d 18 o amplitudes are observed in sites north of the equator indicating a spatial pattern of reduced meridional sst gradient across the equator during the lgm this result is supported by comparison of mg ca sst reconstructions from two sites straddling the equator we interpret the reduction of this gradient during the lgm as evidence for a less intense cold tongue intertropical convergence zone itcz frontal system a more southerly position of the itcz and weaker southeast equatorial trades in the eep indexterms 1620 global change climate dynamics 3309 3339 meteorology and atmospheric dynamics ocean atmosphere interactions 0312 4504 3344 meteorology and atmospheric dynamics paleoclimatology 4231 oceanography general equatorial oceanography 4267 oceanography general paleoceanography keywords cold tongue itcz oxygen isotopes tropical pacific

the provenance of low calcic black shales the elemental concentration of sedimentary rocks depends on the varying reactivity of each element as it goes from the source through weathering deposition diagenesis lithification and even low rank metamorphism however non reactive components of detrital particles ideally are characteristic of the original igneous source and thus are useful in provenance studies to determine the source of detrital granitic and volcanic components of low calcic 1 caco3 marine black shales the concentrations of apparently non reactive i e unaffected by diagenetic redox and or low rank metamorphic processes trace elements were examined using standard trace element discrimination diagrams developed for igneous rocks the chemical data was obtained by neutron activation analyses of about 200 stratigraphically well documented black shale samples from the cambrian through the jurassic a la th sc ternary diagram distinguishes among contributions from the upper and bulk continental crust and the oceanic crust taylor and mclennan 1985 all the low calcic black shales cluster within the region of the upper crust th hf co ternary diagrams also are commonly used to distinguish among the upper and bulk continental crust and the oceanic crust taylor and mclennan 1985 as co is redox sensitive in black shale environments it was necessary to substitute an immobile element i e example rb in the diagram with this substitution of black shales all cluster in the region of the upper continental crust to determine the provenance of the granitic component pearce et al 1984 plots of ta vs yb and rb vs yb ta shows a cluster at the junction of the boundaries separating the volcanic arc granite vag syn collision granite syn colg and within plate granite wpg fields the majority fall within the vag field there are no occurrences of ocean ridge granite org the minimal contribution of basalts to marine black shales is confirmed by the ternary wood diagram th hf 3 ta wood et al 1979 the black shales plot in a cluster in a high th region outside the various basalt fields which suggests contribution from the continental crust

shock metamorphism of enstatite chondrites abstract we have extended the shock metamorphism classification scheme of stoffler et al 1991 to account for shock effects in orthopyroxene and applied it to sixty enstatite chondrites orthopyroxene exhibits the following sequence of progressive shock effects shock stage s1 unshocked sharp optical extinction s2 very weakly shocked undulose extinction s3 weakly shocked development of clinoenstatite lamellae parallel to 100 s4 moderately shocked weak mosaicism s5 strongly shocked strong mosaicism as in the stoffler et al 1991 scheme stage s5 is characterized principally by the solid state transformation of crystalline plagioclase into maskelynite most el3 chondrites exhibit foliations caused by impact deformation of chondrules and metal particles el5 and el6 chondrites are all shock stage s2 because opaque veins are rare in s2 ordinary chondrites it is plausible that the centimeter size kamacite veins in atlanta and blithfield and the 1 6 cm long oldhamite rich vein in jajh deh kot lalu all el6 formed when their hosts were shocked to s3 s5 levels because removal of shock stage s3 s5 features including repair of shock damaged orthopyroxene requires levels of metamorphism comparable to those experienced by petrologic type 5 to 6 chondrites we infer that proto el6 material was shocked to s3 s5 levels prior to peak metamorphism and shocked again to stage s2 after metamorphism overall enstatite chondrites appear to have suffered greater shock damage than ordinary and carbonaceous chondrites

metal size distributions in eh and el chondrites 1 el chondrites exhibit larger average metal grain sizes than eh chondrites a difference attributed by easton 1983 to metamorphic coarsening as most el chondrites are equilibrated and most eh chondrites are unequilibrated in this paper we present metal grain size data for three unequilbrated el3 chondrites pca 91020 alh 85119 mac 88180 and three eh3 chondrites alh 84170 pca 91085 pca 91238 we find that el3 chondrites have larger average metal grain sizes than eh3 chondrites and that grain sizes of the unequilibrated enstatite chondrites are similar to those of equilibrated chondrites of the same class we thus interpret the metal size distributions of enstatite chondrites as primarily reflecting their pre metamorphic distributions shock processing appears to have had only minor influence on metal grain size distributions in these meteorites

low frequency temperature variations from a northern tree ring density network we describe new reconstructions of northern extratropical summer temperatures for nine subcontinental scale regions and a composite series representing quasi northern hemisphere temperature change over the last 600 years these series are based on tree ring density data that have been processed using a novel statistical technique age band decomposition designed to preserve greater long timescale variability than in previous analyses we provide time dependent and timescale dependent uncertainty estimates for all of the reconstructions the new regional estimates are generally cooler in almost all precalibration periods compared to estimates obtained using earlier processing methods particularly during the 17th century one exception is the reconstruction for northern siberia where 15th century summers are now estimated to be warmer than those observed in the 20th century in producing a new northern hemisphere series we demonstrate the sensitivity of the results to the methodology used once the number of regions with data and the reliability of each regional series begins to decrease we compare our new hemisphere series to other published large regional temperature histories most of which lie within the 1 confidence band of our estimates over most of the last 600 years the 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period

antarctic timing of surface water changes off chile and patagonian ice sheet response marine sediments from the chilean continental margin are used to infer millennial scale changes in southeast pacific surface ocean water properties and patagonian ice sheet extent since the last glacial period our data show a clear antarctic timing of sea surface temperature changes which appear systematically linked to meridional displacements in sea ice westerly winds and the circumpolar current system proxy data for ice sheet changes show a similar pattern as oceanographic variations offshore but reveal a variable glacier response time of up to 1000 years which may explain some of the current discrepancies among terrestrial records in southern south america

climate impact of late quaternary equatorial pacific sea surface temperature variations magnesium calcium data from planktonic foraminifera in equatorial pacific sediment cores demonstrate that tropical pacific sea surface temperatures ssts were 2 8 0 7 c colder than the present at the last glacial maximum glacial interglacial temperature differences as great as 5 c are observed over the last 450 thousand years changes in sst coincide with changes in antarctic air temperature and precede changes in continental ice volume by about 3 thousand years suggesting that tropical cooling played a major role in driving ice age climate comparison of sst estimates from eastern and western sites indicates that the equatorial pacific zonal sst gradient was similar or somewhat larger during glacial episodes extraction of a salinity proxy from the magnesium calcium and oxygen isotope data indicates that transport of water vapor into the western pacific was enhanced during glacial episodes

el nino variability in the coastal desert of southern peru during the mid holocene abstract fourteen organic rich sedimentary layers in the deposits at quebrada de los burros in coastal southern peru tacna department lie between two debris flow units interpreted to result from el nino events at 8980 cal yr b p and after 3380 cal yr b p respectively the accumulation of the fine grained and low energy sediments of this deposit during the mid holocene is incompatible with the occurrence of el nino events in this region as these would produce catastrophic flood deposits the occurrence of organic rich sediments and evidence of an enhancement of upwelling strength at this time imply the existence of a permanent water supply resulting from an increased condensation of fog at mid altitudes these results suggest a lower intensity and perhaps a lower frequency of occurrence of the el nino phenomenon during the mid holocene it is precisely during this period that the most important human settlements are found at this site probably indicating the presence of reliable supply of fresh water the chronologies for wetlands in the central south altiplano are out of phase with those indicating increased soil moisture episodes on the coast implying a long term difference in climate between these two regions

the amplitude and phasing of climate change during the last deglaciation in the sulu sea western equatorial pacific 1 variations in tropical sea surface temperature patterns and the phasing relative to climate change in higher latitudes provide insight into the mechanisms of climate change on both orbital and shorter time scales here we present well dated high resolution records of planktonic foraminiferal 18o and mg ca based sst spanning the last deglaciation from the sulu sea located in the western equatorial pacific the results indicate that the last glacial maximum was 2 3 0 5 c cooler than present in the sulu sea with a concomitant decrease in sea surface salinity the similarity between variations in surface salinity in the sulu sea the western and eastern equatorial pacific and the greenland ice core record suggests that the observed changes in salinity reflect large scale rearrangement of atmospheric patterns which were coherent and synchronous throughout the northern hemisphere the results suggest that the glacial equatorial pacific climate was strongly influenced by both tropical and extra tropical forcing although it is not clear whether interannual enso variability is a good analogue of glacial interglacial climate change

tethyan mediterranean organic carbon rich sediments from mesozoic black shales to sapropels the jurassic to holocene record of black shale deposition in the tethys mediterranean region is unrivalled by that of any other ocean basin either in land sections or drill cores the term black shale is used here broadly for sediments with elevated organic carbon concentrations 1 including the pliocene to recent sapropels most of the black shales are devoid of benthonic organisms are laminated and were deposited in distinct rhythms during periods when the deep waters of the ocean basins were anoxic or dysoxic the tethyan black shale records have become essential in studies of the transfer of organic carbon into the sediment record and for astronomical tuning and geological time scales these records have been central in understanding climate control on ocean dynamics and biogeochemical cycles the mesozoic black shales were deposited within well defined time envelopes of around 0 5 to 1 myr these black shales which were confined to certain chronostratigraphic intervals in the jurassic and cretaceous were recognized as expressions of global oceanic anoxic events in the mid 1970s and subsequently named after prominent researchers bonarelli selli goguel the black shale episodes were dated by biostratigraphic methods and by high resolution chemostratigraphy and cyclostratigraphy mesozoic black shales are now interpreted as the oceanographic expression of major perturbations of the global carbon cycle and climate research into younger pliocene to pleistocene sedimentary cycles including black shales termed sapropels exposed in land sections or found in pelagic and hemipelagic marine sediment cores of late quaternary age started in the 1950s main threads pursued from this end of the record were the climatic control of oceanic processes that permitted the development of highly detailed and precise time scales tuned to the astronomical clock of insolation changes palaeoclimate evolution of the circum mediterranean area and biogeochemical dynamics in anoxic basins after 50 years of intensive research the tethyan and mediterranean black shales remain subjects of fascination in earth science tracing the origins and building upon recent progress the current hypotheses on their formation are reviewed here it is a panorama of complex interplays between global and regional tectonics climate dynamics during both ice house and greenhouse states of global and regional climate oceanographic responses to these climate changes and biogeochemical adaptations that were all needed to shape an extraordinary archive of global change in the absence of human activity

properties of chondrules in el3 chondrites comparison with eh3 chondrites and the implications for the formation of enstatite chondrites the study of chondrules provides information about processes occurring in the early solar system in order to ascertain to what extent these processes played a role in determining the properties of the enstatite chondrites the physical and chemical properties of chondrules from three el3 chondrites and three eh3 chondrites have been examined by optical cathodoluminescence cl and electron microprobe techniques properties examined include size texture cl and composition of both individual phases and bulk chondrules the textures distribution of textures and composition of silicates of the el3 chondrules resemble those of eh3 chondrules however the chondrules from the two classes differ in that 1 the size distribution of the el chondrules is skewed to larger values than eh chondrules 2 the enstatite in el chondrules displays varying shades of red cl due to the presence of fine grained sulfides and metal in the silicates and 3 the mesostasis of eh chondrules is enriched in na relative to that of el chondrules the similarities between the chondrules of the two classes suggest similar precursor materials while the differences suggest that there was not a single reservoir of meteoritic chondrules but that their origin was fairly local the differences in the size distribution of chondrules in eh and el chondrites may be explained by aerodynamic and gravitational sorting during accumulation of the meteoric material while differences in cl and mesostasis properties may reflect differences in formation conditions and cooling rate following chondrule formation we argue that our observations are consistent with the formation of enstatite chondrites in a thick dynamic regolith on their parent body

micro x ray diffraction assessment of shock stage in enstatite chondrites abstract a new method for assessing the shock stage of enstatite chondrites has been developed using in situ micro x ray diffraction xrd to measure the full width at half maximum fwhm of peak intensity distributed along the direction of the debye rings or chi angle corresponding to individual lattice reflections in two dimensional xrd patterns this xrd technique differs from previous xrd shock characterization methods it does not require single crystals or powders in situ xrd has been applied to polished thin sections and whole rock meteorite samples three frequently observed orthoenstatite reflections were measured 020 610 and 131 these were selected as they did not overlap with diffraction lines from other phases enstatite chondrites are commonly fine grained stained or darkened by weathering shock induced oxidation and metal sulfide inclusions furthermore most e chondrites have little olivine or plagioclase these characteristics inhibit transmitted light petrography nevertheless shock stages have been assigned macalpine hills mac 02837 el3 s3 pecora escarpment pca 91020 el3 s5 mac 02747 el4 s4 thiel mountains til 91714 el5 s2 allan hills alha 81021 el6 s2 elephant moraine eet 87746 eh3 s3 meteorite hills met 00783 eh4 s4 eet 96135 eh4 5 s2 lewis cliff lew 88180 eh5 s2 queen alexandra range que 94204 eh7 s2 lapaz icefield lap 02225 eh impact melt s1 for the six with published shock stages there is agreement with the published classification fwhm plotted against petrographic shock stage demonstrates positive linear correlation fwhm ranges corresponding to shock stages were assigned as follows s1 3 5 s6 not measured slabs of abee eh impact melt breccia and northwest africa nwa 2212 el6 were examined using xrd alone fwhm values place both in the s2 range consistent with literature values micro xrd analysis may be applicable to other shocked orthopyroxene bearing rocks

modeling el nino and its tropical teleconnections during the last glacial interglacial cycle 1 simulations with the ncar climate system model csm a global coupled ocean atmosphere sea ice model for the last glacial interglacial cycle reproduce recent estimates based on alkenones and mg ca ratios of sea surface temperature sst changes and gradients in the tropical pacific and predict weaker el ninos la ninas compared to present for the holocene and stronger el ninos la ninas for the last glacial maximum lgm changes for the lgm holocene are traced to a weakening strengthening of the tropical pacific zonal sst gradient wind stresses and upwelling and a sharpening weakening of the tropical thermocline results suggest that proxy evidence of weaker precipitation variability in new guinea and ecuador are explained not only by changes in el nino la nina but also changes in the atmospheric circulation and hydrologic cycle index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4267 oceanography general paleoceanography 4522 oceanography physical el nino 3337 meteorology and atmospheric dynamics numerical modeling and data assimilation 9604 information related to geologic time cenozoic citation otto bliesner b l e c brady s i shin z liu and c shields modeling el nino and its tropical teleconnections during the last glacial interglacial cycle geophys res lett 30 23 2198 doi 10 1029 2003gl018553 2003

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

the non trivial problem of meteorite pairing pairing is the procedure of identifying fragments of a single meteorite fall that were separated during atmospheric passage or during terrestrial history by establishing the similarity of two or more meteorite fragments we argue that pairing is governed by two principles that only a single mismatch of properties is required to refute a proposed pairing and that virtually all pairings bear some degree of uncertainty using data distributions for modern falls we take a probability approach to estimate degrees of certainty associated with proposed pairings emphasizing the importance of unusual features for new pairing criteria or new analytical additions to old criteria the degree of variation within individual meteorites must be delineated and the degree of variation within meteorite classes must be quantified criteria for pairing can be divided into 1 parent body history indicators 2 meteoroid space history indicators and 3 terrestrial history indicators included in these categories are 11 specific criteria including petrographic textures mineralogy and mineral composition terrestrial age estimates cosmic ray exposure ages and natural thermoluminescence tl levels not all criteria are applicable to all meteorite types about 2275 pairings suggested in the literature have been subjected to this analysis many literature pairings especially those involving common meteorite types bear large uncertainties due to lack of data

enhanced preservation of marine derived organic matter in cenomanian black shales from the southern angola basin black shales possessing high concentrations of organic carbon1 were deposited in many parts of the proto south atlantic ocean during the cretaceous period2 the way such sediments accumulated is not fully understood but is likely to have occurred through a combination of low oxygen availability and abundant supply of organic matter thin centimetre thick layers of black shales are commonly interbedded with thicker layers of organic carbon deficient green claystones as found in strata of aptian to coniacian age at deep sea drilling project dsdp site 530 in the southern angola basin3 and elsewhere these differences in carbon content and colour reflect the conditions of deposition and possibly variations in the supply of organic matter4 5 we have compared using organic geochemical methods the compositions of organic matter in three pairs of closely bedded black and green cenomanian claystones obtained from site 530 kerogen analyses and distributions of biological markers show that the organic matter of the black shales is more marine and better preserved than that of the green claystones

a shock metamorphic model for silicate darkening and compositionally variable plagioclase in ck and ordinary chondrites abstract silicate darkening in ordinary chondrites oc is caused by tiny grains of metallic fe ni and troilite occurring mainly within curvilinear trails that traverse silicate interiors and decorate or in some cases cut across silicate grain boundaries highly shocked oc characterized by olivine grains with undulose to mosaic extinction tend to have greater degrees of silicate darkening than lightly shocked oc this indicates that silicate darkening is probably a result of shock metamorphism the low fe fes eutectic temperature 988 c renders metal and troilite susceptible to melting and mobilization during shock heating a few oc also contain thin melt veins of chromite this implies that localized shock temperatures reached 1635 c in these meteorites silicate darkening is also evident in ck carbonaceous chondrites where magnetite and pentlandite grains form analogous curvilinear trails it is possible that magnetite and pentlandite in ck chondrites were mobilized during shock metamorphism and dispersed through silicate interiors alternatively metal and troilite may have been dispersed and then transformed into magnetite and pentlandite during subsequent oxidation of these chondrites unshocked oc tend to have plagioclase with uniform compositions shocked oc tend to have plagioclase with more variable albeit still stoichiometric compositions the low impedance of plagioclase to shock compression makes it particularly susceptible to melting and mobilization this is consistent with the molten appearance of plagioclase in highly shocked oc e g rose city and paragould ck chondrites also have compositionally variable plagioclase the common association of silicate darkening with compositionally variable plagioclase is consistent with the hypothesis that both are products of shock metamorphism some ck and oc chondrites exhibit light shock effects in olivine that are consistent with equilibrium peak shock pressures that are too low to account for the silicate darkening or opaque shock veins in these meteorites therefore the olivine in these chondrites may have been annealed after intense shock produced these effects a few ck chondrites that contain olivine with undulose or mosaic extinction e g lew87009 and eet83311 may have been shocked again after annealing

global surface temperatures over the past two millennia 1 we present reconstructions of northern and southern hemisphere mean surface temperature over the past two millennia based on high resolution proxy temperature data which retain millennial scale variability these reconstructions indicate that late 20th century warmth is unprecedented for at least roughly the past two millennia for the northern hemisphere conclusions for the southern hemisphere and global mean temperature are limited by the sparseness of available proxy data in the southern hemisphere at present

evidence for stronger el nino southern oscillation enso events in a mid holocene massive coral we present a 47 year long record of sea surface temperature sst derived from sr ca and u ca analysis of a massive porites coral which grew at 4150 calendar years before present b p in vanuatu southwest tropical pacific ocean mean sst is similar in both the modern instrumental record and paleorecord and both exhibit el nino southern oscillation enso frequency sst oscillations however several strong decadal frequency cooling events and a marked modulation of the seasonal sst cycle with power at both enso and decadal frequencies are observed in the paleorecord which are unprecedented in the modern record

the cenomanian turonian anoxic event in europe an organic geochemical study received 14 october 1988 revised 14august 1989 accepted 31 august 1989 thirty two sediment samples mainly black shales from cenomanian turonian organic rich sequences in europe and tunisia were examined by organic geochemical techniques the results of molecular analyses indicate that all the black shales are dominated by marine derived organic matter from both algal and bacterial sources with only a minor terrestrial component oxygen deficient conditions prevailed during the deposition of the black shales although interbedded non laminated organic lean horizons indicate that oxygen deficiency was intermittent at some sites the presence of 17cx h 18c h 211 h 28 30 bisnorhopane in some black shale samples from shallow shelf areas may be evidence for severely oxygen depleted conditions possibly in association with upwelling and an oxygen minimum layer overall the organic geochemical data are in agreement with a depositional model whereby an oxygen minimum layer developed in response to high surface productivity this oxygen minimum layer may have extended over a considerable thickness of the water column 1000 m in some areas we report the first observation of an a ring methylated c29 hopane namely methyl 17e h 18 h 211 h 28 30 bisnorhopane the environmental significance of this compound cannot yet be assessed although in the sample in which it occurs the direct parallel between the carbon number distribution of the hopanes and that of the methylhopanes suggests either that they have the same biological origin or that methylation of hopanoids may have occurred possibly during diagenesis

determining chondrule size distributions from thin section measurements a method is described for correcting thin section derived chondrule sizes for three common sources of bias 1 the non equatorial sectioning of chondrules 2 the non zero thickness of thin sections and 3 the unequal probability of sectioning different size chondrules application of the correction procedure to chondrule data from co chondrites results in a reduction of mean and median chondrule diameters an increase in minimum diameters and transforms nearly log normal distributions to distributions that conform to a weibull probability function the weibull function form of co chondrule size distributions is similar to the form of chondrule distributions in ordinary chondrites obtained by disaggregation analyses

orbital and suborbital climate variability in the sulu sea western tropical pacific 1 a detailed record of planktic 18o from a sediment core in the sulu sea located between the south china sea and the western pacific warm pool reveals that for the past 400 kyr 1 kyr 1000 years 18o variability on orbital timescales is similar to that caused by changes in ice volume alone this result indicates that in the sulu sea temperature driven changes in planktic 18o on orbital times scales were generally compensated for by the effects of sea level and changes in seasonal monsoon intensity on the local freshwater budget as well as by other changes in the tropical hydrologic cycle and their attendant effects on surface water 18o increased freshening of the western tropical pacific warm pool is reminiscent of la nina conditions however we argue that the mean tropical climate state was not more la nina like than today on broader spatial scales suborbital variability occurred in the sulu sea throughout the past 400 kyr suggesting little sensitivity to ice volume or to glacial interglacial changes in tropical hydrology variations on 4 10 kyr timescales appear to be linked to those in the north atlantic region suggesting a common forcing of that variability we suggest that sulu sea salinity variations were a response to suborbital climate variability in the north atlantic region transmitted via changes in the intensity of the east asian summer monsoon we suggest that a north atlantic origin of that tropical suborbital variability can be reconciled with weak glacial amplification in the tropics if the tropical response is nonlinear

sedimentation and accumulation of organic carbon in the angola basin and on walvis ridge preliminary results of deep sea drilling project leg 75 leg 75 investigated two periods of accumulation of sediments rich in organic matter in the upper cretaceous section more than 200 beds of gray or black shale and mudstone rich in organic carbon as much as 18 are intercalated within a sequence of red and green mudstones that contain less than 1 organic carbon the black shale beds are 1 to 62 cm thick and constitute less than 10 of the section in the 20 cores in which they were recovered both turbiditic and pelagic processes were involved in the deposition of these black shale beds a combination of factors including increased productivity and warm saline bottom waters probably resulted in a delicate balance between oxic and anoxic conditions that periodically favored the accumulation and preservation of organic matter in the basinal sediments the pliocene pleistocene sediments are thoroughly bioturbated and there is no evidence of persisting anoxia in the overlying water in spite of the high organic carbon content of the sediments these two different periods of accumulation of organic matter both resulted in significant concentration of organic carbon 2 maximum 18 but the pliocene pleistocene sediments contain the greater mass of organic carbon per unit volume of the solid sediment