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

visualisation and quantification of cv chondrite petrography using micro tomography abstract micro computed tomography is a non destructive technique that allows the study of 3d meteorite petrography the technique produces a unique and instructive visualisation of the meteorite for quantifying its components we studied the overall petrography of the two cv chondrites allende and mokoia to constrain their formation histories a set of movies and stereographic images detail the 3d petrography component modal abundances agree with previous reports and modal abundance differences between allende and mokoia support the chondrule matrix complementarity and that chondrules and matrix formed from the same chemical reservoir we identified two types of chondrules a normal type and one where a normal type i or ii chondrule is almost completely encapsulated by an opaque rich layer this layer was probably acquired during a late stage condensation process the appearance of opaques in chondrules and matrix is different not supporting a genetic relationships between these low abundances of compound chondrules 1 75 vol in allende and 2 50 vol in mokoia indicate low chondrule densities and or low relative component velocities in chondrule formation regions porosities on a scale

the glauconite fe illite fe smectite problem a critical review iron silicate minerals are a significant component of sedimentary systems but their modes of formation remain controversial our analysis of published data identifies end member compositions and mixtures and allows us to recognize controls of formation of different mineral species the compositional fields of glaucony fe illite fe al smectites are determined in the m 4si vs fe sum of octahedral cations m interlayer charge solid solutions could exist between these phases the fe al and fe rich clay minerals form two distinct solid solutions the earliest phases to be formed are fe al smectites or berthierine depending on the sedimentation rate reductive microsystems appear in the vicinity of organic debris in unconsolidated sediments the fe is incorporated first in pyrite and then in silicates after oxidation potassium ions diffuse from the sea water sediment interface if not interrupted the diffusion process is active until reaction completion is reached i e formation of fe illite or glauconite or a mineral assemblage berthierine nontronite according to the available al ion amounts in the microsystem mixed layer minerals are formed when the diffusion process is interrupted because of sedimentation compaction or cementation despite the common belief of their value as palaeoenvironment indicators these minerals can form in a variety of environments and over a period of millions of years during sediment burial

pedogenic modification of loess significance for palaeoclimatic reconstructions abstract this review considers the role of pedogenic processes in modifying wind blown dust loess concentrating particularly on the ways that resulting properties may be interpreted as indicators of past climatic conditions and changes emphasis is placed on the sequences of palaeosols developed within loess deposits that are frequently regarded as some of the best terrestrial equivalents of marine sediment records of long term global climatic change a palaeosol is generally interpreted in terms of the broad pedogenic processes and environments assumed to be currently responsible for that type of soil forming at the present surface even the very presence of a palaeosol may have palaeoclimatic significance however in that it is often taken to indicate a period of relative land surface stability and warmer and or moister conditions between cold and or arid phases of loess accumulation in reality it may be more useful to consider many loess palaeosol sequences in terms of changing balances between pedogenesis and loess accumulation over geological time in most regions it seems that the balance swings towards pedogenesis during interglacials or interstadials when sediment supply and transport are limited and the climate is warmer and or wetter where accumulation rates are still appreciable during these soil forming intervals however the soils and palaeosols may be accretionary with surface build up keeping pace with pedogenesis welding may also occur where covering sediments are insufficiently thick to isolate an underlying palaeosol from the effects of pedogenesis active at a new land surface further complications occur due to reworking of palaeosols and syndepositional pedogenic alteration of loess units generally such pedocomplexes can only be deciphered if the different pedogenic geomorphic and sedimentary processes are identified and ordered within a pedosedimentary reconstruction a recent trend has been to treat some loess palaeosol sequences as quasi continuous time series particularly when comparing depth functions of climatic proxy properties such as magnetic susceptibility and grain size with marine and ice core isotope curves

paleoclimatic implications of holocene plant remains from the sierra bacha sonora mexico abstract a total of 93 plant taxa were identified from 11 packrat neotoma sp midden samples from the sierra bacha on the coast of the gulf of california near puerto libertad sonora mexico nine indurated samples have radiocarbon dates ranging from 9970 to 320 yr b p sonoran desertscrub was present on rocky slopes throughout the holocene early holocene assemblages dominated by fouquieria columnaris boojum tree reflect vegetation and climate more like modern baja california with greater winter rainfall and cooler summers middle holocene vegetation was essentially modern with modest indications of greater monsoonal rainfall even though cold water upwelling locally inhibits summer precipitation the results are similar to all previous midden reconstructions of early and middle holocene climates in the sonoran desert but contradict general atmospheric circulation model simulations

wet conditions during the last glaciation in the chihuahuan desert alta babicora basin mexico abstract a 467 cm core b94 3 from the alta babicora basin in the chihuahuan desert northern mexico 29 n 108 w documents lake level and climate changes over the past ca 65 000 yr chronological control is provided by four accelerator mass spectrometry 14 c dates and five u series dates on diatom silica the core has been analyzed for magnetic susceptibility loss on ignition carbonate content sediment chemistry and mineralogy and pollen and diatom composition the basin was occupied by a deep freshwater lake throughout the late pleistocene which based on shoreline evidence was at least 19 m deeper than today the lake shallowed after ca 57 000 yr b p high variability typified the period between 54 600 and 38 000 yr b p probably with periodic desiccation and deflation a deep water lake was reestablished after 38 000 yr b p and persisted until ca 29 000 yr b p shallowing occurred through the last glacial maximum although the lake was still deeper than at present the pleistocene holocene transition was marked by a distinctive change in the diatom flora there is no diatom record for much of the holocene but other proxies indicate generally dry stable catchment conditions the babicora record is more akin to those from the southwest united states than it is to central america and northern south america wet conditions in the late pleistocene are attributed to winter rainfall from midlatitudes in contrast to the modern tropical summer rainfall regime

late quaternary paleohydrological conditions in the drylands of northern mexico a summer precipitation proxy record of the last 80 cal ka bp abstract we present a late quaternary multi proxy record from the paleolake babicora in order to understand the influence of north american monsoon nam and westerly winter storms on the paleohydrological budget of the drylands of northern mexico stratigraphy 14 c chronology mineral magnetism and geochemistry of the 976 cm long sediment core were used to reconstruct the hydrological conditions over the last 80 cal ka bp the inverse relationship between the proxy records of runoff into babicora and winter precipitation in southwestern usa indicate the minimal influence of westerly winter storms at 29 n during the last glacial period westerly winds transported minimally chemically altered sediments from the dry watershed during the cold stadials and the basin received more than average runoff as a result of a stronger nam during the warm interstadials the highest pluvial discharge occurred prior to 58 cal ka bp and terrestrial plants became the major contributor to organic matter deposited between 71 and 53 cal ka bp over the last 40 cal ka bp the high amplitude fluctuations in runoff and lake water salinity mirrored an unstable summer rainfall regime

a new kind of primitive chondrite allan hills 85085 abstract allan hills 85085 is a chemically and mineralogically unique chondrite whose components have suffered little metamorphism or alteration this chondrite is unique because it has fewer and smaller chondrules 4 wt mean diameter 16 m than any other chondrite more metallic fe ni 36 and lithic and mineral silicate fragments 56 and a lower abundance of troilite 2 and volatiles most chondrules are cryptocrystalline or glassy and are depleted in volatiles some small chondrules are also very depleted in refractory lithophiles matrix lumps 4 partly resemble ci and cm matrices and may be foreign to the parental asteroid despite these differences the components of alh 85085 have some features common to most type 2 and the least metamorphosed type 3 chondrites metallic fe ni grains that contain 0 1 1 wt cr si and p fe fe mg values of olivines pyroxenes and chondrules are concentrated in the range 1 6 at with a few percent in the range 7 30 porphyritic chondrules are chondritic in composition except for their low volatile abundances thus the components of alh 85085 probably have similar origins to those of components in other chondrites and their properties largely reflect nebular not asteroidal processes the bulk composition of alh 85085 fits none of the nine groups of chondrites it is richer in fe 1 4 ci levels when normalized to si and poorer in na and s 0 1 0 2 ci than other chondrites low volatile concentrations are due to a low matrix abundance and loss of volatiles during or prior to chondrule formation not to volatile loss during metamorphism chondrule textures imply extensive heating of chondrule melts above the liquidus consistent with loss of volatiles from small volumes of melt during chondrule formation the small size of chondrules is partly due to extensive fragmentation by impacts which may have occurred on the parent asteroid or in the solar nebula collisions between chondrule precursor aggregates in the nebula could also be responsible for the small sizes of chondrules assuming that alh 85085 is a representative sample of an asteroid its properties lend support to models for the origins of the earth eucrite parent body and volatile poor iron meteorites that invoke chondritic planetesimals depleted in volatiles the existence of alh 85085 and kakangari suggests that the nine chondrite groups may provide a remarkably poor sample of the primitive chondritic material from which the asteroids formed certain similarities between alh 85085 and bencubbin and weatherford suggest that the latter two primitive meteorites may actually be chondrites with even higher metal abundances 50 60 wt and very large partly fragmented chondrules

geochemical and magnetic evidence of change from winter to summer rainfall regimes at 9 2 cal ka bp in northwestern mexico abstract we present a multiproxy record from magnetic susceptibility loss on ignition and chemical analysis of a sediment core collected from cienega las taunas a peatland in the northwest of the sierra madre occidental in mexico the oldest sediments were dated to 12 850 cal yr bp the relatively cool and wet younger dryas yd shows higher concentrations of tio2 and chemical index of alteration cia suggesting higher runoff into the cienega the transition from the yd to the early holocene is marked by a decrease in susceptibility pointing to weakened erosional processes during a period of closed forest vegetation suggesting a predominantly winter rainfall regime a change in seasonality likely occurred after ca 9200 cal yr bp as shown by 1 an increase in most major elements suggesting increased dust fallout from distant sources and 2 more pollen assemblages from temperate vegetation pine oak forest we related these changes to increased summer rains and monsoons in nw mexico a possible hiatus or a very slow sedimentation period occurred during the middle holocene at this site finally high frequency changes in sediment properties were especially observed in the late holocene suggesting contrasting sediment inputs controlled by an unstable water table and or related to enso

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

geochemistry of the peat bog at etang de la gruere jura mountains switzerland and its record of atmospheric pb and lithogenic trace metals sc ti y zr and ree since 12 370 14 c yr bp a 650 cm core from a swiss bog represents 12 370 14c years of peat accumulation and provides the first complete record of atmospheric pb deposition for the entire holocene excess non atmospheric sr in the peat was calculated by normalizing sr sc to crustal abundance this was used to differentiate between the ombrogenic section of the bog in which inorganic solids are supplied exclusively by atmospheric deposition and the minerogenic zone where mineral water interactions contribute metals to the peat while sediment dissolution contributes significantly to the sr inventory of the minerogenic section of the peat profile there is no measurable effect of this process on the pb burden isotopic analyses 204pb 206pb 207pb 208pb show that effectively all of the pb in the peat profile was supplied exclusively from the atmosphere to separate natural and anthropogenic pb sc was selected over ti y zr hf and ree as a conservative reference element which is supplied by soil dust aerosols derived from rock weathering enrichment factors ef were calculated using the pb sc ratio in the peat samples normalized to the natural background pb sc which is found in peats dating from 8030 to 5320 14c yr bp the results show that anthropogenic source have dominated the supply of atmospheric pb to the peat core continuously since 3000 14c yr bp the aerosols supplied to the bog can be divided into 3 classes a pre anthropogenic older than 3000 14c yr bp with pb ef 1 194 b pre industrial dating from 3000 240 14c yr bp with pb ef 2 but 20 and 206pb 207pb 1 179 elevated soil dust fluxes are observed at 5320 8230 and 10 590 14c yr bp the latter corresponds to the younger dryas aluminum zr hf and ree sc ratios also are elevated at the same depths suggesting differences in particle size wind strength or source regions pre anthropogenic aerosols deposited since 8230 14c yr bp reveal pb sc which is significantly higher and 206pb 207pb which is less radiogenic than during the early holocene while the trend toward increasing pb sc could be due to chemical weathering and soil development this could not explain the shift in pb isotopic composition the changes which took place at 8230 14c bp therefore may be related to a large scale climatic reorganization which at present is poorly understood

acfer 182 and paired samples an iron rich carbonaceous chondrite similarities with alh85085 and relationship to cr chondrites three samples of a new fe rich chondrite were found in the sahara in 1990 and 1991 acfer 182 acfer 207 acfer 214 the samples are paired and the meteorite will be designated as acfer 182 the chondrite is chemically texturally and mineralogically similar to the allan hills meteorite alh85085 one important difference between the two meteorites is the smaller average chondrule size in alh85085 the major components of acfer 182 in decreasing abundance are 1 1 highly altered by terrestrial weathering matrix 2 2 mineral and polymineralic silicate fragments and aggregates 3 3 chondrule fragments 4 4 chondrules 5 5 metal 6 6 fine grained dark inclusions the abundance of chondrules is lower and the average chondrule size 90 m smaller than in most other chondrites chondrule fragments are often so large that they cannot be derived from the present chondrule population apparently size sorting has prevented accumulation of the larger parent chondrules several spectacular ca al rich inclusions were found rich in ca dialuminate hibonite or zr y sc bearing phases the chemical composition of acfer 182 and of alh85085 are almost indistinguishable major chemical signatures are 1 1 uniform enrichment of fe and other nonvolatile metals relative to ci chondrites by about 70 2 2 absence of enrichment in refractory lithophiles characteristic of most type 2 and 3 carbonaceous chondrites 3 3 strong depletion of volatile and moderately volatile elements based on the oxygen isotopic composition the chemical composition and the abundances of chondrules and matrix acfer 182 should be classified as a carbonaceous chondrite considering their affinity to carbonaceous chondrites and their high bulk iron content the two meteorites acfer 182 and alh85085 are designated as ch chondrites there are mineralogical and chemical similarities among acfer 182 alh85085 and cr chondrites which distinguish these meteorites from other types of carbonaceous chondrites 1 1 low feo contents of olivine and pyroxene and correspondingly high metal contents 2 2 high cr content in olivine 3 3 abundant fine grained dark inclusions 4 4 abundant ca dialuminate caal4o7 in cais 5 5 similarities in oxygen isotopic composition 6 6 low contents of moderately volatile elements 7 7 low refractory element contents 8 8 presence of a unique component of subsolar rare gases these observations suggest similar conditions of formation for the components of these meteorites a single common parent body is unlikely in view of the differences in chemical composition and in the size distribution of individual components

origin of sio2 rich components in ordinary chondrites abstract silica rich objects are common minor components in ordinary chondrites oc occurring as fragments and as chondrules their typical paragenesis is orthopyroxene sio 2 with bulk sio 2 65 wt and occasionally with additional olivine and or spinel individual silica rich components src have previously been studied in various types of ocs although there is only one comprehensive study of these objects by brigham et al brigham c a murrell m t yabuki h ouyang z el goresy a 1986 silica bearing chondrules and clasts in ordinary chondrites geochim cosmochim acta 50 1655 1666 several different explanations of how srcs formed have been published the main question is how silica enrichment was achieved because ci chondritic atomic mg si ratio is 1 07 and as a consequence only olivine and pyroxene but no free silica should be stable there are two basic possibilities for the sio 2 enrichment 1 a redox mechanism or magmatic fractionation on the parent body and 2 fractional condensation or recycling of chondrule mesostasis in the solar nebula to better constrain the origin of these objects we measured major and rare earth elements in srcs of various types of ordinary chondrites and in addition we studied silica polymorphism in these objects using an in situ micro raman technique bulk chondrule compositions define mixing lines between the compositions of olivine and pyroxene the srcs extend these lines to an sio 2 end member in contrast magmatic trends grossly deviate from these mixing lines concentrations of cao al 2 o 3 and ree in the pyroxenes of the srcs are low 0 01 to 1 ci and the ci normalized ree patterns are virtually flat typical of bulk chondrules but untypical of magmatic trends we therefore conclude that sio 2 rich objects are not of magmatic origin they are the result of fractional condensation in the solar nebula the silica in srcs occurs mainly as tridymite and sometimes as cristobalite or in very rare cases as quartz some sio 2 phases yielded a yet unknown micro raman spectrum which we were unable to identify the often chondrule like shape of srcs as well as the presence of high temperature sio 2 polymorphs lead to the following model for the origin of srcs formation of sio 2 rich precursors in the solar nebula by fractional condensation reheating to temperatures between 1140 and 1968 k thereby forming the srcs probably during the chondrule forming process followed by rapid cooling

sea surface temperature records of termination 1 in the gulf of california challenges for seasonal and interannual analogues of tropical pacific climate change 1 centennial scale records of sea surface temperature and opal composition spanning the last glacial maximum and termination 1 circa 25 6 ka are presented here from guaymas basin in the gulf of california through the application of two organic geochemistry proxies the u37k index and the tex86h index we present evidence for rapid stepped changes in temperatures during deglaciation these occur in both temperature proxies at 13 ka 3 c increase in 270 years 10 0 ka 2 c decrease over 250 years and at 8 2 ka 3 c increase in 200 years an additional rapid warming step is also observed in tex86h at 11 5 ka in comparing the two temperature proxies and opal content we consider the potential for upwelling intensity to be recorded and link this millennial scale variability to shifting intertropical convergence zone position and variations in the strength of the subtropical high the onset of the deglacial warming from 17 to 18 ka is comparable to a southern hemisphere signal although the opal record mimics the ice rafting events of the north atlantic heinrich events neither the modern seasonal cycle nor el nino southern oscillation patterns provide valid analogues for the trends we observe in comparison with other regional records fully coupled climate model simulations confirm this result and in combination we question whether the seasonal or interannual climate variations of the modern climate are valid analogues for the glacial and deglacial tropical pacific

alh85085 a unique volatile poor carbonaceous chondrite with possible implications for nebular fractionation processes abstract allan hills 85085 is a unique chondrite with affinities to the al rais renazzo clan of carbonaceous chondrites its constituents are less than 50 m in mean size chondrules and microchondrules of all textures are present nonporphyritic chondrules are unusually abundant the mean compositions of porphyritic nonporphyritic and barred olivine chondrules resemble those in ordinary chondrites except that they are depleted in volatile elements ca al rich inclusions are abundant and largely free of nebular alteration they comprise types similar to those in cm and co chondrites as well as unique types calcium dialuminate occurs in several inclusions metal silicate and sulfide compositions are close to those in cm co chondrites and al rais and renazzo c1 chondrite clasts and metal rich reduced clasts are present but opaque matrix is absent siderophile abundances in alh85085 are extremely high e g fe si 1 7 solar and volatiles are depleted e g na si 0 25 solar s si 0 03 solar nonvolatile lithophile abundances are similar to those in al rais renazzo and cm and co chondrites alh85085 agglomerated when temperatures in the nebula were near 1000 k in the same region where renazzo al rais and the ci chondrites formed agglomeration of high temperature material may thus be a mechanism by which the fractionation of refractory lithophiles occurred in the nebula chondrule formation must have occurred at high temperatures when clumps of precursors were small after agglomeration alh85085 was annealed and lightly shocked c1 and other clasts were subsequently incorporated during late stage brecciation

heavily hydrated lithic clasts in ch chondrites and the related metal rich chondrites queen alexandra range 94411 and hammadah al hamra 237 fine grained heavily hydrated lithic clasts in the metal rich cb chondrites queen alexandra range que 94411 and hammadah al hamra 237 and ch chondrites such as patuxent range pat 91546 and allan hills alh 85085 are mineralogically similar suggesting genetic relationship between these meteorites these clasts contain no anhydrous silicates and consist of framboidal and platelet magnetite prismatic sulfides pentlandite and pyrrhotite and fe mn mg bearing ca carbonates set in a phyllosilicate rich matrix two types of phyllosilicates were identified serpentine with basal spacing of similar to0 73 nm and saponite with basal spacings of about 1 1 1 2 nm chondrules and feni metal grains in cb and ch chondrites are believed to have formed at high temperature 1300 k by condensation in a solar nebula region that experienced complete vaporization the absence of aqueous alteration of chondrules and metal grains in cb and ch chondrites indicates that the clasts experienced hydration in an asteroidal setting prior to incorporation into the ch and cb parent bodies the hydrated clasts were either incorporated during regolith gardening or accreted together with chondrules and feni metal gains after these high temperature components had been transported from their hot formation region to a much colder region of the solar nebula

mid wisconsin sediment record from baldwin lake reveals hemispheric climate dynamics southern ca usa abstract recently acquired cores from baldwin lake southern california document the first regional terrestrial based evidence for glacial period millennial to orbital scale climate variability eight ams 14c dates on a 14 m core from the lake s present day depocenter provide initial age control over the interval from 20 000 to 48 000 years before present bp a linear age model extends the record to a minimum age of 65 000 years bp a combination of lithologic description and one cm contiguous sedimentological analyses indicates significant changes in the dominant type of depositional environment over the length of the record at millennial to orbital time scales as a first order interpretation the dominant sedimentary environments alternate between permanent and ephemeral lake systems orbital scale forcing includes long term winter summer insolation variability and its modulation of winter storm tracks and the north american monsoon respectively millennial scale variability is attributed to extra tropical ocean atmosphere dynamics akin to historical interdecadal pacific climate variability in addition there is notable correspondence to the independently dated greenland ice core record gisp 2 at millennial to orbital time scales suggesting that baldwin lake contains in addition to its local regional record a hemispheric record of climate change similar to owens pyramid and summer lake of central and northwestern north america an initial comparison to gisp2 18o ice indicates that north atlantic interstadials correlate to permanent lake environments in coastal southwestern north america these results present a notable conflict with the proposed super enso phenomenon in the western tropical pacific these results also highlight the necessity for acquiring additional high resolution glacial period records for resolving questions of spatial temporal phasing of late quaternary climates

last glacial hydrological variations at the southern margin of sub tropical north america and a regional comparison geochemical and magnetic characteristics of sediments from paleolake la salada provide information about the last glacial hydrological conditions at the southern margin of sub tropical north america proxy records of runoff lake water salinity brine composition and provenance of organic matter are based on multi element concentration magnetic susceptibility mineralogy carbonate content and c n relation runoff was higher than average during ca 43 36k cal a bp and at ca 33k cal a bp a shift in mineralogical assemblage from calcite protodolomite halite to protodolomite gaylussite suggests that the brine composition changed from ca mg na hco3 cl to mg na ca hco3 because of more runoff after the desiccation event at ca 28k cal a bp in general organic matter deposited during the intervals of more runoff and deeper lake conditions were sourced from lacustrine phytoplankton the contribution of terrestrial vegetation dominates during the intervals of reduced runoff comparison with other records provides a regional perspective of moisture variability humid conditions at ca 33k cal a bp could be due to more summer precipitation in northern mexico as well as increased winter rainfall in the south western usa

evidence for pacific modulated precipitation variability during the late holocene from the southwestern usa 1 the mechanisms driving late holocene drought cycles in the western united states are not well known due to the general scarcity of long term high resolution absolutely dateable proxies for precipitation in continental interiors here we show that late holocene precipitation variability in the southwestern united states has been caused by changes in the pacific ocean we present a stalagmite based annually resolved moisture record that indicates large shifts from pluvial to drought conditions alternating with periods of dampened near average precipitation over the last 3000 years significant spectral peaks at decadal scale 20 50 70 80 year frequencies likely correspond to modern frequencies of the pacific decadal oscillation or a low frequency component of the el nino southern oscillation and are dominant during episodes of large precipitation shifts overall pluvial conditions punctuated by severe droughts may have challenged the adaptive capacities of emerging agrarian communities of ancestral americans

shock melts in que 94411 hammadah al hamra 237 and bencubbin remains of the missing matrix we have studied the cb carbonaceous chondrites queen alexandra range que 94411 hammadah al hamra hh 237 and bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone iron nickel metal and chondrule silicates are the main components in these meteorites these high temperature components are held together by shock melts consisting of droplets of dendritically intergrown fe ni metal sulfide embedded in silicate glass which is substantially more feo rich 3040 wt than the chondrule silicates feo 5 wt fine grained matrix material which is a major component in most other chondrite classes is extremely scarce in que 94411 and hh 237 and has not been observed in bencubbin this material occurs as rare hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts and cl we infer that hydrated fine grained material compositionally similar to these matrix lumps was originally present between the fe ni metal grains and chondrules but was preferentially shock melted other shock related features in que 94411 hh 237 and bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments

climate and ocean variability during the middle and late holocene recorded in laminated sediments from alfonso basin gulf of california mexico a laminated sequence core bap96 cp 24 38 12 n 110 33 24 w 390 m depth from the alfonso basin in bay of la paz southern gulf of california contains a record of paleoceanographic and paleoclimatic changes of the past 7900 yr radiolarian assemblages and magnetic susceptibility are used as proxies of oceanographic and climatic variability the records provide a regional scenario of the middle and late holocene suggesting two major climatic regimes and several millennial scale events conditions relatively warmer and drier than today occurred from 7700 to 2500 cal yr bp promoting the intensification of evaporation processes and the prevalence of the gulf of california water in the basin these conditions correlate with strong droughts in the middle holocene of north america and with minimal incursion of tropical waters into the gulf of california proxies indicate a warm scenario and the dominance of the equatorial surface water in the alfonso basin from 2400 to 700 cal yr bp suggesting the intensification of enso cycles a climatic signal between 1038 and 963 cal yr bp may be correlated with global signal of the medieval warm period several cooling events are recognized at 5730 3360 2700 1280 and 820 cal yr bp and are associated with intensification of northwest winds leading to upwellings and enhanced productivity in the basin

quantifying the error of 2d bulk chondrule analyses using a computer model to simulate chondrules simchon bulk chondrule compositions are important to many questions in cosmochemistry however the number of available bulk chondrule data sets is still small a main reason for this is the difficulties of determining bulk chondrule compositions a commonly used technique is to obtain 2d bulk chondrule compositions from meteorite sections this technique has an error that we quantify here for the first time using a mathematical model of a chondrule called simchon the theoretically calculated errors are compared to errors that we determined from serial sectioning of eight efremovka chondrules the errors obtained from both approaches are in excellent agreement proving that our mathematical model produces reliable errors that can be assigned to 2d bulk chondrule compositions these errors allow a much better interpretation of 2d bulk chondrule data we provide a table that contains typical errors for 2d bulk compositions of porphyritic chondrules the errors are in the range of 1 30 relative this should be acceptable for many problems in cosmochemistry the effect of a chemical layering inside chondrules and the occurrence of a rim around them as well as the occurrence of opaque and other accessory phases have been studied a spreadsheet is provided that enables the calculation of errors for any desired chondrule mineral composition bo chondrules have a negligible error but it is impossible to provide reasonable error estimates for bo chondrules with an igneous rim radial pyroxene chondrules have negligible errors

physical properties of chondrules in different chondrite groups implications for multiple melting events in dusty environments chondrite groups cv ck cr with large average chondrule sizes have low proportions of rp plus c chondrules high proportions of enveloping compound chondrules high proportions of chondrules with thick igneous rims and relatively low proportions of type i chondrules containing sulfide in contrast chondrite groups cm co oc r eh el with smaller average chondrule sizes have the opposite properties equilibrated ck chondrites have plagioclase with relatively low na equilibrated oc r eh and el chondrites have more sodic plagioclase enveloping compound chondrules and chondrules with igneous rims formed during a remelting event after the primary chondrule was incorporated into a dustball repeated episodes of remelting after chondrules were surrounded by dust would tend to produce large chondrules rp and c chondrules formed by complete melting of their precursor assemblages remelting of rp and c chondrules surrounded by dust would tend to produce porphyritic chondrules as small dust particles mixed with the melt providing nuclei for crystallizing phenocrysts this process would tend to diminish the numbers of rp and c chondrules correlations among these chondrule physical properties suggest that chondrite groups with large chondrules were typically surrounded by thick dust rich mantles that formed in locally dusty nebular environments chondrules that were surrounded by thick dust mantles tended to cool more slowly because heat could not quickly radiate away slow cooling led to enhanced migration of sulfide to chondrule surfaces and more extensive sulfide evaporation these chondrules also lost na the plagioclase that formed from equilibrated ck chondrites was thus depleted in na

primordial oxygen isotope reservoirs of the solar nebula recorded in chondrules in acfer 094 carbonaceous chondrite abstract highly precise and accurate ion microprobe analyses of oxygen three isotope ratios in chondrules from the acfer 094 one of the most primitive carbonaceous chondrites show that chondrules preserve evidence for oxygen isotope heterogeneity in chondrule forming regions of the solar nebula identical 17 o values in most minerals and glass within each chondrule indicate that the oxygen isotope ratio in chondrule melt did not change during or after crystallization nearly half of the chondrules studied contain small amounts of olivine grains that have an oxygen isotope anomaly relative to other minerals and glass in the same chondrule most chondrules in acfer 094 can be classified into two oxygen isotope groups 17 o 2 and 17 o 5 indicating that the final melting of chondrules occurred within two distinct oxygen isotope reservoirs probably representing the local protoplanetary disk immediately before planetesimal formation one of these reservoirs 17 o 2 is observed from chondrules in other carbonaceous chondrites and from crystalline silicates in comet wild 2 suggesting that crystalline silicates formed in an oxygen isotope reservoir of 17 o 2 were widely distributed in the outer asteroid belt and throughout the outer solar nebula oxygen three isotope ratios of minerals in chondrules from acfer 094 are distributed along a newly defined primitive chondrule minerals pcm line which has slope 1 17 o 0 987 0 013 18 o 2 70 0 11 and intersects the terrestrial fractionation line at 18 o 5 8 0 4 these data are distinct from and plot between the ccam and young and russell lines the pcm line is interpreted to represent the mixing trend of extreme oxygen isotope reservoirs in the early solar system that were the primary oxygen isotope reservoir of solids that accreted to form planets including the earth

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

orbital pacing and ocean circulation induced collapses of the mesoamerican monsoon over the past 22 000 y the dominant controls on global paleomonsoon strength include summer insolation driven by precession cycles ocean circulation through its influence on atmospheric circulation and sea surface temperatures however few records from the summer north american monsoon system are available to test for a synchronous response with other global monsoons to shared forcings in particular the monsoon response to widespread atmospheric reorganizations associated with disruptions of the atlantic meridional overturning circulation amoc during the deglacial period remains unconstrained here we present a high resolution and radiometrically dated monsoon rainfall reconstruction over the past 22 000 y from speleothems of tropical southwestern mexico the data document an active last glacial maximum 18 24 cal ka b p monsoon with similar 18o values to the modern and that the monsoon collapsed during periods of weakened amoc during heinrich stadial 1 ca 17 ka and the younger dryas 12 9 11 5 ka the holocene was marked by a trend to a weaker monsoon that was paced by orbital insolation we conclude that the mesoamerican monsoon responded in concert with other global monsoon regions and that monsoon strength was driven by variations in the strength and latitudinal position of the intertropical convergence zone which was forced by amoc variations in the north atlantic ocean the surprising observation of an active last glacial maximum monsoon is attributed to an active but shallow amoc and proximity to the intertropical convergence zone the emergence of agriculture in southwestern mexico was likely only possible after monsoon strengthening in the early holocene at ca 11 ka

the cr chondrite clan implications for early solar system processes in this paper we review the mineralogy and chemistry of calcium aluminum rich inclusions cais chondrules feni metal and fine grained materials of the cr chondrite clan including cr ch and the metal rich cb chondrites queen alexandra range 94411 hammadah al hamra 237 bencubbin gujba and weatherford the members of the cr chondrite clan are among the most pristine early solar system materials which largely escaped thermal processing in an asteroidal setting bencubbin weatherford and gujba may be exceptions and provide important constraints on the solar nebula models these constraints include 1 multiplicity of cai formation 2 formation of cals and chondrules in spatially separated nebular regions 3 formation of cais in gaseous reservoir s having o 16 rich isotopic compositions chondrules appear to have formed in the presence of o 16 poor nebular gas 4 isolation of cais and chondrules from nebular gas at various ambient temperatures 5 heterogeneous distribution of al 26 in the solar nebula and 6 absence of matrix material in the regions of cai and chondrule formation

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

microstructure and thermal history of metal particles in ch chondrites we have studied metal microstructures in four ch chondrites patuxent range pat 91546 allan hills alh 85085 acfer 214 and northwest africa nwa 739 to examine details of the thermal histories of individual particles four types of metal particles are common in all of these chondrites zoned and unzoned particles probably formed as condensates from a gas of chondritic composition in a monotonic cooling regime as has been shown previously we have demonstrated that these particles were cooled rapidly to temperatures below 500 k after they formed and that condensation effectively closed around 700 k zoned and unzoned particles with exsolution precipitates predominantly high ni taenite have considerably more complex thermal histories precipitates grew in reheating episodes but the details of the heating events vary among individual grains reheating temperatures are typically in the range 800 1000 k reheating could have been the result of impact events on the ch parent body some particles with precipitates may have been incorporated into chondrules with further brief heating episodes taking place during chondrule formation in addition to the four dominant types of metal particles rare ni rich metal particles and si rich metal particles indicate that the metal assemblage in ch chondrites was a mixture of material that formed at different redox conditions metal in ch chondrites consists of a mechanical mixture of particles that underwent a variety of thermal histories prior to being assembled into the existing brecciated meteorites

the magnitudes of millennial and orbital scale climatic change in eastern north america during the late quaternary abstract temporal differences among fossil pollen samples from eastern north america provide a measure of the amount of climatic change that occurred on a broad spatial scale since the last glacial maximum 21 000 cal yr bp square chord distances scds quantify the difference between pollen samples and thus represent the potential magnitude of climatic change underlying the pollen record the magnitude varied significantly among time scales with small changes common at centennial to millennial scales and large changes common at multi millennial i e orbital scales scds measured across 3000 yr intervals averaged 0 20 and often exceeded the maximum difference expected from samples collected within the same biome 0 15 scds across individual millennia were smaller and averaged 0 08 scds across the millennia at the beginning 13 000 12 000 cal yr bp and end 12 000 11 000 cal yr bp of the younger dryas chronozone 12 900 11 600 cal yr bp however averaged 0 20 and 0 18 respectively these rapid step changes large at sub millennial scales equal about 5 25 of the total glacial interglacial transition large magnitude progressive changes in insolation ice sheet extent and atmospheric composition parallel the dominant trends in the scd data which show that progressive change comprises the first order climatic pattern of the holocene rather than stable or oscillatory patterns

formation of chondrules and matrix in kakangari chondrites abstract the study of chondritic meteorites and their components allows us to understand processes and conditions in the protoplanetary disk chondrites with high and about equal proportions of chondrules and matrix are ideal candidates to not only study the formation conditions of chondrules but also the relationship between these two major components an important question is whether these formed in the same or in separate reservoirs in the protoplanetary disk so far such studies have been mainly restricted to carbonaceous chondrites we here expand these studies to the k kakangari like chondrite grouplet these have various distinctive properties but the abundance of major components chondrules and matrix is similar to other primitive meteorites we obtained a comprehensive petrographic and chemical dataset of kakangari and lewis cliff 87232 chondrules and matrix chondrules in kakangari show a large compositional scatter supporting material addition to chondrules during their formation contrary to almost all other chondrite groups the majority of kakangari chondrules are not mineralogically zoned however kakangari chondrules were likely initially zoned but then lost this zonation during chondrule remelting and fragmentation average compositions of bulk chondrules matrix and bulk kakangari are identical and approximately solar for mg si this might indicate the formation of chondrules and matrix from a common reservoir and would agree with findings from carbonaceous and rumuruti chondrites chondrules and matrix in most chondrite groups were not transported through the protoplanetary disk and then mixed together rather these major components are genetically related to each other and formed in the same reservoir

ferrous silicate spherules with euhedral iron nickel metal grains from ch carbonaceous chondrites evidence for supercooling and condensation under oxidizing conditions the ch carbonaceous chondrites contain a population of ferrous fe fe mg approximate to 0 1 0 4 silicate spherules chondrules about 15 30 mum in apparent diameter composed of cryptocrystalline olivine pyroxene normative material sio2 rich glass and rounded to euhedral fe ni metal grains the silicate portions of the spherules are highly depleted in refractory lithophile elements cao al2o3 and tio2 0 04 wt and enriched in feo mno cr2o3 and na2o relative to the dominant volatile poor magnesian chondrules from ch chondrites the fe fe mg ratio in the silicate portions of the spherules is positively correlated with fe concentration in metal grains which suggests that this correlation is not due to oxidation reduction or both of iron feosil fe met during melting of metal silicate solid precursors rather we suggest that this is a condensation signature of the precursors formed under oxidizing conditions each metal grain is compositionally uniform but there are significant intergrain compositional variations about 8 18 wt ni 0 09 wt cr and a sub solar co ni ratio the precursor materials of these spherules were thus characterized by extreme elemental fractionations which have not been observed in chondritic materials before particularly striking is the fractionation of ni and co in the rounded to euhedral metal grains which has resulted in a co ni ratio significantly below solar the liquidus temperatures of the euhedral fe ni metal grains are lower than those of the coexisting ferrous silicates and we infer that the former crystallized in supercooled silicate melts the metal grains are compositionally metastable they are not decomposed into taenite and kamacite which suggests fast postcrystallization cooling at temperatures below 970 k and lack of subsequent prolonged thermal metamorphism at temperatures above 400 500 k

terminal particle from stardust track 130 probable al rich chondrule fragment from comet wild 2 abstract a 4 6 m terminal particle from stardust track 130 named bidi is composed of a refractory assemblage of fo 97 olivine al and ti bearing clinopyroxene and anorthite feldspar an 97 mineralogically bidi resembles a number of components found in primitive chondritic meteorites including al rich chondrules plagioclase bearing type i ferromagnesian chondrules and amoeboid olivine aggregates aoas measured widths of augite pigeonite lamellae in the clinopyroxene indicate fast cooling rates suggesting that bidi is more likely to be a chondrule fragment than an aoa bulk element concentrations including an al 2 o 3 content of 10 2 wt further suggests that bidi is more akin to al rich rather than ferromagnesian chondrules this is supported by high anorthite content of the plagioclase feldspar overall bulk composition and petrogenetic analysis using a cosmochemical al 2 o 3 ca 2 sio 4 mg 2 sio 4 phase diagram measured minor element abundances of individual minerals in bidi generally support an al rich chondrule origin but are not definitive between any of the object types oxygen isotope ratios obtained from olivine minor high ca pyroxene fall between the tf and ccam lines and overlap similar minerals from chondrules in primitive chondrites but are generally distinct from pristine aoa minerals oxygen isotope ratios are similar to some minerals from both al rich and type i ferromagnesian chondrules in unequilibrated carbonaceous enstatite and ordinary chondrites although no single piece of evidence uniquely identifies bidi as a particular object type the preponderance of data including mineral assemblage bulk composition mineral chemistry inferred cooling rates and oxygen isotope ratios suggest that bidi is more closely matched to al rich chondrules than aoas or plagioclase bearing type i ferromagnesian chondrules and likely originated in a chondrule forming region in the inner solar system

application of proton microprobe data to trace element partitioning in volcanic rocks abstract in situ proton microprobe analyses are used to calculate partition coefficients d for mn fe cu zr ga rb sr y zr ba and pb and more restricted data for ni ge nb and as in pyroxenes olivines biotites hornblendes feldspars and fe ti oxides from 32 volcanic rocks all with glassy matrices these range from leucitite and picritic basalt to high silica and peralkaline rhyolites data for pigeonite aenigmatite allanite and leucite are included where these occur as additional phases coefficients have also been rayleigh fractionation corrected this correction being significant for those elements with high coefficients in specific mineral phases and where pronounced crystallisation has occurred partition coefficients are presented as plots vs nbo t ratios of coexisting glasses melts negative correlations are observed based on log transformed data for mn zn fe and y pyroxenes mn zn and fe olivines mn and zn fe ti oxides nb y sr and zr hornblendes sr plagioclase and ba cu zn and fe feldspars these element partition coefficients are not correlated with bulk mineral compositions zr and ga are not correlated with either glass or mineral compositions except zr in hornblende the correlations support the role of melt polymerisation in controlling element partitioning at least for those elements listed although possible effects of temperature cannot be uniquely isolated in the present data set the coefficients presented here are generally consistent with published values for equivalent rock compositions one difference being that some of the higher published coefficients e g sr in plagioclase are not matched in the present data set from a practical viewpoint the relative regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic modelling procedures

paleoceanographic history of the guaymas basin gulf of california during the past 15 000 years based on diatoms silicoflagellates and biogenic sediments high resolution records of calcium carbonate biogenic opal diatoms and silicoflagellates from western guaymas basin gravity core ggc55 and piston core jpc56 and eastern guaymas basin dsdp site 480 reveal a complex paleoceanographic history of the central gulf of california during the past 15 000 years prior to 6 2 ka the eastern and western guaymas basin proxy records were remarkably similar after conditions similar to those of today during the bolling allerod the younger dryas yd saw a major drop in diatom production coincident with increased calcium carbonate and tropical microfossils suggestive of el nino like conditions biosiliceous productivity began increasing during the latter part of the yd but it was only during the earliest holocene 11 6 to 11 0 ka that conditions similar to those of the bolling allerod returned to the central gulf between around 11 0 and 6 2 ka tropical diatoms and silicoflagellates were virtually absent from the central gulf as relatively cooler and fresher surface waters resembling those of the modern northern gulf were present in the central gulf beginning at about 6 2 ka tropical diatoms and silicoflagellates began increasing in the central gulf and coccoliths returned to western gulf sediments the onset of modern day monsoon conditions in the american southwest required the presence of warm ssts in the northern gulf which probably did not occur until after about 5 4 ka when tropical diatoms and silicoflagellates became relatively common in the central gulf modern east west contrasts which arise from late winter early spring coastal upwelling on the mainland side and lower diatom productivity on the western side of the gulf commenced between 6 2 and 5 4 ka possibly due to a shift in the direction of late winter early spring winds more towards the southeast or down the axis of the gulf this proposed wind shift might have ultimately been due to a late holocene strengthening of enso like conditions in the eastern equatorial pacific published by elsevier b v

mineralogically zoned chondrules in ordinary chondrites as evidence for open system chondrule behaviour abstract chondrules are a major component of chondritic meteorites understanding their formation conditions provides fundamental insights about how the early solar system formed and evolved we studied the textures of 650 chondrules from all three groups h l ll of ordinary chondrites in 2 dimensional 2d sections through the meteorites about 40 of the chondrules are mineralogically zoned they consist of an olivine rich core which is surrounded by a low ca pyroxene rich rim chondrules sectioned through their low ca pyroxene rim do not appear as zoned chondrules hence considering such sectioning effects their true fraction might be as high as 50 mineralogical zonation is therefore a typical chondrule texture in basically all ordinary chondrites and records a fundamental process during chondrule formation chondrules were open systems and initially olivine rich chondrules reacted with their surrounding gas to form low ca pyroxene rims zoned and unzoned chondrules have the same range of bulk compositions thus ordinary chondrite chondrules were likely affected by two sequential episodes in the first episode gaseous sio was added to all chondrules thereby forming low ca pyroxene rims around all chondrules in the second episode only a portion of the chondrules were reheated thereby remelting and homogenizing their initial pyroxene rims but retaining their bulk compositions it is therefore likely that all chondrules in ordinary chondrites were affected by gas melt interactions during their formation open system exchange is consistent with previous studies of chondrule formation and can explain many chondrule textures and bulk chondrule compositional variations in single meteorites hence the open system behaviour recorded in zoned chondrules provides a pivotal constraint on chondrule formation conditions

formation of niningerite by silicate sulfidation in eh3 enstatite chondrites unequilibrated eh chondrites contain silica bearing chondrules with abundant niningerite mg fe mn s and troilite fes distinguishing them from the silica bearing chondrules in ordinary and carbonaceous chondrites the conventional explanation for the origin of niningerite and oldhamite cas is that they are condensates from c rich nebular gas however models of condensation from a solar gas with an elevated c o ratio predict mineralogy that is inconsistent with petrographic observations we report petrographic and chemical evidence from 45 silica bearing chondrules from the eh3 chondrites sahara 97072 and alan hills 84170 for formation of niningerite and oldhamite by sulfidation of ferromagnesian silicates the results indicate extensive thermal processing of the chondrules including melting before during and after sulfidation bulk compositions of chondrules exhibiting varied degrees of sulfidation suggest that depletion of mg and enrichment of fe mn and na accompany the reactions sulfidation of feo bearing silicates which formed at oxidizing conditions at least iw 3 can occur with exposure to a h poor c and s rich gaseous reservoir 6 8 fo2 log units below the iw buffer at temperatures high enough for partial melting of silicates physicochemical analysis of mineral reactions inferred from sulfidized chondrules suggests a melted metal sulfide assemblage in a h poor environment is capable of generating sufficient s vapor to drive sulfidation the reaction of silicates with the s gas will result in progressive extraction of fe ca and mg into sulfides with the stoichiometric amounts of silica either reacting with olivine to form enstatite or when olivine is exhausted precipitating as free silica the sulfidation environment drastically increases mg volatility resulting in evaporative loss until saturation in the ambient gas is reached the newly formed mg rich niningerite will tend to reach equilibrium composition by losing mg and gaining fe plus mn from the ambient gas consistent with the observed chemical fractionation in the sulfidized chondrules therefore sulfidation of ferromagnesian silicates can also explain the low bulk mg si ratios of the enstatite chondrites if the sulfidizing gas was lost before accretion of the enstatite chondrite parent body

isotopic evidence for younger dryas aridity in the north american midcontinent determining the impact of the younger dryas yd climate event on the unglaciated north american midcontinent has proved difficult due to a scarcity of suitable paleoclimate proxies here we present a well dated carbon isotope 13c record from a buried soil sequence in southwestern missouri which reveals a large isotopic excursion during the yd chronozone in this region of the modern prairie forest border the 13c signature of soil organic matter is a reliable indicator of past climatic change because 13c values are controlled primarily by the relative abundance of c3 and c4 plants which is tied to the environmental setting between ca 13 200 and 11 900 yr ago the abundance of c4 grasses increased by upwards of 50 of the total biomass indicating expansion of grassland most likely driven by increased aridity during this period environmental gradients in the midcontinent must have been very steep because at the same time that a c4 rich prairie existed in southwestern missouri spruce forests grew in iowa illinois and ohio

variability in the mean latitude of the atlantic intertropical convergence zone as recorded by riverine input of sediments to the cariaco basin venezuela abstract changes in the strength and position of the intertropical convergence zone itcz are an important component of climate variability in the tropical atlantic the cariaco basin located on the northern margin of venezuela is sensitive to tropical atlantic climate change and its sediments provide a record of past itcz behavior today the cariaco basin experiences two distinct seasons that reflect the annual migration of the atlantic itcz between january and march when the itcz lies south of the equator northeasterly trade winds sit directly over cariaco basin and strong coastal upwelling and dry conditions dominate beginning in june july as the itcz moves north local rainfall reaches a maximum and the upwelling diminishes or disappears here we summarize new and previously published data on the river derived terrigenous fraction of cariaco basin sediments as well as comparisons to other paleoclimate records which together suggest a coherent climatologic response in the tropical atlantic triggered by a pattern of itcz migration that mimics the seasonal cycle during periods of cooler north atlantic ssts on time scales ranging from the little ice age to the younger dryas to the cold stadials of the last glacial decreased detrital delivery to cariaco basin from local rivers suggests a southward shift in the mean latitudinal position of the itcz during warm interstadials and periods of holocene and deglacial warmth northward shifts in itcz position and its belt of convective rainfall are inferred from increased detrital delivery to the basin whether the rapid shifts in itcz position and precipitation recorded by cariaco basin sediments and other regional records reflect a response to forcing originating in the high latitude atlantic or to forcing potentially sourced in the tropics is a key question yet to be fully answered

late pleistocene paleohydrology near the boundary of the sonoran and chihuahuan deserts southeastern arizona usa abstract ground water discharge gwd deposits form in arid environments as water tables rise and approach or breach the ground surface during periods of enhanced effective precipitation where preserved these deposits contain information on the timing and elevation of past ground water fluctuations here we report on the investigation of a series of gwd deposits that are exposed in discontinuous outcrops along a 150 km stretch of the san pedro valley in southeastern arizona near the boundary of the sonoran and chihuahuan deserts chronologic isotopic geochemical faunal assemblage ostracodes and gastropods and sedimentological evidence collectively suggest that the elevation of the regional water table in the valley rose in response to a change in climate 50 ka ago and remained relatively high for the next 35 ka before falling during the bolling allerod warm period rebounding briefly during the younger dryas cold event and falling again at the onset of the holocene the timing of these hydrologic changes coincides closely with variations in 18o values of calcite from a nearby speleothem to the west and changes in lake levels at pluvial lake cochise to the east thus in southeastern arizona the assumption that changes in climate are reflected in all aspects of the hydrologic cycle of a region simultaneously is validated the timing of these changes also broadly coincides with variations in the gisp2 18o record which supports the hypothesis that atmospheric teleconnections existed between the north atlantic and the deserts of the american southwest during the late pleistocene

environmental setting of the early irrigation in oasisamerica paleopedological evidences from the alluvial palaeosols in la playa sonoran desert abstract environmental conditions and human landscape interaction during the onset of the irrigated agriculture in oasisamerica are not clear yet in la playa site sonora nw mexico the evidences of sedentary population land cultivation and water management developed after the end of altithermal period around 4500 yr bp include more than 550 archaeological features including hundreds of human cremations and a net of buried artificial channels these findings were closely associated with several palaeosol levels alternating with sediments within a large alluvial fan we studied micromorphological features physical characteristics color texture rock magnetic properties and composition of organic matter from three profiles maravillas zanja and cuatro suelos to reconstruct pedogenesis and sedimentary environment of the palaeosol sequences additional paleoenvironmental information was extracted from pollen assemblages extracted from the cuatro suelos profile the results pointed to a long period of geomorphic and climatic stability in the early middle holocene marked by a well developed red cambisol this period was followed by an unstable interval around 4 5 kyr bp marked by severe erosion of earlier soil profiles and sedimentation of different kind channel floodplain and fluvio eolian later synsedimentary fluvisols were formed showing signs of predominantly arid pedogenesis interrupted by occasional flooding indicators of human impact were also encountered we conclude that the shift to irrigated agriculture as the main subsistence activity occurred during the period of major climatic and geomorphic fluctuation and then irrigation developed further under dry environment with limited water resources during the late holocene

a new metal rich chondrite grouplet abstract a new grouplet of primitive metal rich chondrites here called the cb c carbonaceous b bencubbinite chondrites has been recognized it includes bencubbin weatherford hammadah a1 hamra hh 237 and queen alexandra range que 94411 paired with que 94627 their mineral compositions as well as their oxygen and nitrogen isotopic compositions indicate that they are closely related to the cr and ch chondrites all of which are members of the more inclusive cr clan cb chondrites have much greater metal silicate ratios than any other chondrite group widely increasing the range of metal silicate fractionation recorded in solar nebular processes they also have the greatest moderately volatile lithophile element depletions of any chondritic materials metal has compositional trends and zoning patterns that suggest a primitive condensation origin in contrast with metal from other chondrite groups cb chondrites as well as other cr clan chondrites have much heavier nitrogen higher 15n 14n than that in other chondrite groups the primitive characteristics of the cb chondrites suggest that they contain one of the best records of early nebular processes another chondrite grosvenor mountains 9555 1 is petrographically similar to the cb chondrites but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir

sulfur and sulfides in chondrules abstract the nature and distribution of sulfides within type i po pop and pp chondrules of the carbonaceous chondrite vigarano cv3 have been studied by secondary electron microscopy and electron microprobe they occur predominantly as spheroidal blebs composed entirely of low ni iron sulfide troilite fes or troilite magnetite but in less abundance in association with metallic fe ni beads in opaque assemblages troilites are mainly located within the low ca pyroxene outer zone and their amounts increase with the abundance of low ca pyroxene within chondrules suggesting co crystallization of troilite and low ca pyroxene during high temperature events we show that sulfur concentration and sulfide occurrence in chondrules obey high temperature sulfur solubility and saturation laws depending on the f s 2 and f o 2 of the surrounding gas and on the melt composition mainly the feo content sulfur dissolved in chondrule melts may eventually reach a concentration limit the sulfur content at sulfide saturation scss at which an immiscible iron sulfide liquid separates from the silicate melt the occurrence of both a silicate melt and an immiscible iron sulfide liquid is further supported by the non wetting behavior of sulfides on silicate phases in chondrules due to the high interfacial tension between their precursor iron sulfide liquid droplets and the surrounding silicate melt during the high temperature chondrule forming event the evolution of chondrule melts from po to pp towards more silicic compositions very likely due to high p sio g of the surrounding nebular gas induces saturation of fes at much lower s content in pp than in po chondrules leading to the co crystallization of iron sulfides and low ca pyroxenes conditions of co saturation of low ca pyroxene and fes are only achieved in non canonical environments characterized by high partial pressures of sulfur and sio and redox conditions more oxidizing than iw 3 fe and s mass balance calculations also suggest the occurrence of an external source of iron very likely gaseous during chondrule formation we therefore propose that enrichments in sulfur and other volatile and moderately volatile elements from po to pp type i bulk chondrule compositions towards chondritic values result from progressive reaction between partially depleted olivine bearing precursors and a volatile rich gas phase

evidence from lake lisan of solar influence on decadal to centennial scale climate variability during marine oxygen isotope stage 2 documentation of short term climate variability during the glacial period has been limited by the availability of well dated high resolution archives here we present a paleoclimate reconstruction from varved lacustrine sediments of lake lisan dead sea rift for ca 26 2 17 7 calendar ka the age is based on a floating varve chronology anchored to radiometric dates our reconstruction indicates that small ice rafting events a b c and d as well as heinrich events in the north atlantic are associated with the eastern mediterranean arid intervals study of seasonal sublaminae yields evidence of several additional decadal to century scale arid events that correlate with cooler temperatures at higher latitudes analyses in the frequency domain indicate the presence of periodicities centered at 1500 yr 500 yr 192 yr 139 yr 90 yr and 50 60 yr suggesting a solar forcing on climate

early holocene juniper woodland and chaparral taxa in the central baja california peninsula mexico abstract a packrat midden located in the sierra san francisco baja california sur mexico dating to ca 10 200 14c yr b p contains remains of california juniper juniperus californica and other taxa now associated with southern california chaparral california juniper does not occur in the sierra san francisco today although relict populations of a few chaparral taxa still occur at higher elevations this midden record documents the early holocene occurrence of baja california coniferous woodland and chaparral vegetation far south of its present distribution or its previously known extent from other fossil records based on modern climatic tolerances of california juniper and other taxa central baja california experienced a mild mediterranean type climate at least 5 6 c cooler than the climate of today with at least twice the winter precipitation the region now receives

the conditions of chondrule formation part i closed system abstract mixing was an important process in the early solar nebula and is often used as an argument to explain the compositional scatter among chondrules mm sized once molten silicate spherules in chondritic meteorites if it is hypothesized that chondrules only acted as closed systems and the scatter in chondrule bulk chemical compositions is only the result of mixing heterogeneous precursor grains the basic components of chondrules it is in turn possible to determine the sizes of the precursor grains using statistical calculations in order to reproduce the observed compositional scatter in chondrules not more than 10 precursor grains should contribute to a single chondrule each with a diameter of several 100 m this finding has important implications for the conditions of chondrule formation and replaces the so far widely accepted model that chondrules formed from fine grained dust balls chondrules rather formed from coarse grained precursor aggregates with variable amounts of m fine matrix material as a consequence only chondrite matrix or interstellar material winnows as precursor material large grains of variable composition serving as precursor grains must have been formed prior to chondrule formation chondrules probably have not been their immediate precursors as only 1 2 chondrule recycling steps would have homogenized bulk chondrule compositions chondrule recycling can therefore only have occurred to a limited extent chondrule formation needed at least three steps 1 production of large and heterogeneous chondrule precursor grains 2 agglomeration of large precursor grains and fine grained precursors into aggregates 3 formation of chondrules during transient heating events al rich chondrules can in this context be explained by the admixture of cais to either chondrule precursors or a population of existing chondrules

response of the north american monsoon to regional changes in ocean surface temperature 1 the north american monsoon nam an onshore wind shift occurring between july and september has evolved in character during the holocene largely due to changes in northern hemisphere insolation published paleoproxy and modeling studies suggest that prior to 8000 cal years bp the nam affected a broader region than today extending westward into the mojave desert of california holocene proxy sst records from the gulf of california goc and the adjacent pacific provide constraints for this changing nam climatology prior to 8000 cal years bp lower goc ssts would not have fueled northward surges of tropical moisture up the goc which presently contribute most of the monsoon precipitation to the western nam region during the early holocene the north pacific high was further north and ssts in the california current off baja california were warmer allowing monsoonal moisture flow from the subtropical pacific to take a more direct northwesterly trajectory into an expanded area of the southwestern u s west of 114 w a new upwelling record off southwest baja california reveals that enhanced upwelling in the california current beginning at 7500 cal year bp may have triggered a change in nam climatology focusing the geographic expression of nam in the southwest usa into its modern core region east of 114 w in arizona and new mexico holocene proxy precipitation records from the southwestern u s and northwestern mexico including lakes vegetation pollen and caves are reviewed and found to be largely supportive of this hypothesis of changing holocene nam climatology

a nebula setting as the origin for bulk chondrule fe isotope variations in cv chondrites abstract we combined micro computer tomography with fe and si isotope measurements of mokoia allende and grosnaja chondrules ten mokoia chondrules contain 0 9 to 11 8 vol opaque phases metal sulfide and 6 allende chondrules contain 0 0 to 6 6 vol opaque phases hence the fe isotope composition of many chondrules is dominated by the fe isotope composition of their opaque phases we studied fe isotopes of 35 bulk chondrules the range is different for each of the three meteorites studied and largest for allende with 56 fe ranging from 0 82 to 0 37 six out of seven chondrules analysed for their si isotope composition in mokoia and grosnaja have similar 29 si of around 0 12 one anomalous chondrule in mokoia has a 29 si of 0 58 we exclude isotopically heterogeneous chondrule precursors and different isotopic chondrule reservoirs as the source of the observed fe isotope variation among bulk chondrules we conclude that the observed bulk chondrule fe isotope variation is the result of evaporation and re condensation processes in a nebula setting with high dust densities required to explain the comparatively low isotope fractionations subsequent parent body alteration slightly overprinted this pre accretionary fe isotope variation

refractory inclusions and aluminum rich chondrules in sayh al uhaymir 290 ch chondrite petrography and mineralogy here we report the petrography mineralogy and bulk compositions of ca al rich inclusions cals amoeboid olivine aggregate aoa and al rich chondrules arcs in sayh al uhaymir sau 290 ch chondrite eighty two cals 0 1 of the section surface area were found they are hibonite rich 9 grossite rich 18 melilite spinel rich 48 fassaite spinel rich 15 and fassaite rich 10 refractory inclusions most cals are rounded in shape and small in size average 40 mu m they are more refractory than those of other groups of chondrites cals in sau 290 might have experienced higher peak heating temperatures which could be due to the formation region closer to the center of protoplanetary disk or have formed earlier than those of other groups of chondrites in sau 290 refractory inclusions with a layered texture could have formed by gas solid condensation from the solar nebula and those with an igneous texture could have crystallized from melt droplets or experienced subsequent melting of pre existing condensates from the solar nebula one refractory inclusion represents an evaporation product of pre existing refractory solid on the basis of its layered texture and melting temperature of constituting minerals only one aoa is observed 75 mu m across it consists of olivine al diopside anorthite and minor spinel with a layered texture cals and aoa show no significant low temperature aqueous alteration arcs in sau 290 consist of diopside forsterite anorthite al enstatite spinel and mesostasis or glass they can be divided into diposide rich al enstatite rich glass rich and anorthite rich chondrules bulk compositions of most arcs are consistent with a mixture origin of cals and ferromagnesian chondrules anorthite and al enstatite do not coexist in a given arc implying a kinetic effect on their formation

ion microprobe analyses of oxygen three isotope ratios of chondrules from the sayh al uhaymir 290 ch chondrite using a multiple hole disk the ion microprobe is the only technique capable of determining high precision stable isotope ratios in individual tiny extraterrestrial particles 100 lm in diameter but these small samples present special analytical challenges we produced a new sample holder disk with multiple holes three holes and seven holes in which epoxy disks containing a single unknown sample and a standard grain are cast and polished performance tests for oxygen two isotope analyses using san carlos olivine standard grains show that the new multiple hole disks allow accurate analysis of tiny particles if the particles are located within the 500 lm and 1 mm radius of the center of holes for seven hole and three hole disks respectively using the new seven hole disk oxygen three isotope ratios of eight magnesian cryptocrystalline chondrules approximately 100 lm in diameter from the sayh al uhaymir sau 290 ch chondrite were analyzed by ion microprobe at the university of wisconsin five out of eight chondrules have nearly identical oxygen isotope ratios d 17 o 2 2 0 6 2sd which is consistent with those of magnesian cryptocrystalline chondrules in ch cb and cb chondrites suggesting a genetic relationship i e formation by a common possibly impact heating event the other three chondrules have distinct oxygen isotope ratios d 17 o values from 6 4 to 2 2 given that similar variation in d 17 o values was observed in type i porphyritic chondrules in a ch cb chondrite the three chondrules may have formed in the solar nebula similar to the type i porphyritic chondrules

compositions of unzoned and zoned metal in the cbb chondrites hammadah al hamra 237 and queen alexandra range 94627 the cbb chondrites are rare primitive metal rich meteorites that contain several features including zoned metal that have previously been interpreted as evidence for origins in the solar nebula we have measured concentrations of ni cu ga ru pd ir and au within both zoned and unzoned metal grains in the cbb chondrites hammadah al hamra hah 237 and queen alexandra range que 94627 using laser ablation inductively coupled plasma mass spectrometry the refractory elements ni ru and ir are enriched in the grain cores relative to the rims in the zoned metal all refractory elements are uniform across the unzoned metal grains at concentrations that are highly variable between grains the volatile elements cu ga and au are usually depleted relative to chondritic abundances and are most often uniform within the grains but are sometimes slightly elevated at the outermost rim the pd abundances are nearly uniform at close to chondritic abundances in all of the metal grains a condensation origin is inferred for both types of metal the data support a model in which the zoned metal formed at high temperatures in a relatively rapidly cooling nebular gas and the unzoned metal formed at lower temperatures and at a lower cooling rate the cbb metal appears to have formed by a process very similar to that of the ch chondrites but the cbb meteorite components experienced even less thermal alteration following their formation and are among the most primitive materials known to have formed in the solar nebula

allan hills 85085 a subchondritic meteorite of mixed nebular and regolithic heritage abstract allan hills 85085 is a tiny 12 g meteorite whose closest relatives are the renazzo and al rais chondrites and the subchondritic host material in the bencubbin meteorite its bulk composition is generally chondritic but siderophile abundances are tens of per cent higher and abundances of moderately volatile elements factors of 2 4 lower than in known authentic chondrites in addition to this unusual composition alh85085 has an extremely small particle size metal that shows no evidence of metamorphic equilibration and a very high abundance of pyroxenitic lithic particles chondrules are rare and small most are pyroxenitic and chemically similar to the lithic particles previous workers have inferred that most or all of these strange features of alh85085 resulted from nebular processes but we suggest that melting vaporization outgassing condensation and size sorting in a cloud of impact ejecta offer a more viable alternative until additional similar materials have been discovered and characterized it seems safest to infer nebular properties and processes on the basis of the record in chondrites regarding which there is little doubt that nebular processes were dominant

the geochemical record of the last 17 000 years in the guaymas basin gulf of california abstract sediments deposited on the western slope of the guaymas basin in the central gulf of california are composed predominantly of detrital clastic material and biogenic silica biopal with minor organic material average of 2 8 organic carbon and calcium carbonate the caco 3 is derived from calcareous plankton and is highly variable ranging from 0 to 16 in general the caco 3 content of the sediments varies inversely with the biopal content reflecting the relative abundance of calcareous and siliceous plankton in the photic zone siliceous plankton dominate when winds are predominantly out of the northwest producing strong upwelling calcareous plankton indicates weak southeasterly winds that bring warm tropical pacific surface water into the gulf based mainly on relative abundances of biopal and caco 3 the sediments deposited over the last 17 000 years in the western guaymas basin can be divided into five intervals in general the sediments in the intervals with high biopal and low caco 3 are laminated but this is not always true unlike most other continental margins of the world with well developed oxygen minimum zones where highest concentrations of organic carbon and redox sensitive trace metals occur in laminated sediments the laminated sediments on the anoxic slope of the western guaymas basin do not always have the highest concentrations of organic carbon and trace metals such as mo and cd

primitive feni metal grains in ch carbonaceous chondrites formed by condensation from a gas of solar composition some feni metal grains similar to 150 mu m in apparent diameter in ch carbonaceous chondrites are concentrically zoned in ni similar to 5 10 wt co 0 2 0 4 wt and cr 0 3 0 8 wt silicon is present at the similar to 0 1 wt level these observations are consistent with predicted gas solid condensation from a gas of solar composition at temperatures of similar to 1370 1270 k and total pressure of similar to 10 4 bar estimates of feni metal grain growth and cooling rates in this temperature range are consistent with brief and localized thermal episodes in the solar nebula compositionally similar feni metal grains have also been reported in cr and bencubbin like chondrites because feni metal is highly susceptible to secondary alteration i e metamorphism melting oxidation the observed feni metal condensates in ch bencubbin like and cr chondrites indicate that these meteorites experienced no thermal processing after their lithification and thus are among the most primitive meteorites in our collections

geochemistry mineralogy and geochemical mass balance on major elements in two peat bog profiles jura mountains switzerland abstract the mineralogical and chemical composition of peats from two sphagnum bogs in the franches montagnes region jura mountains switzerland were compared the peats in the top 102 cm at etang de la gruere egr represent 2110 years of peat formation compared with 1730 years for the first 84 cm at tourbiere de genevez tge scandium was used as a conservative tracer to distinguish between the primary sources of major elements to the bogs atmospheric deposition of soil derived aerosols egr and tge physical incorporation of elements along with mineral matter from the underlying sediments tge and adsorption or complexation by the peat following diffusion from groundwater tge these results support previous findings which showed that the egr core 102 cm is exclusively ombrogenic compared with tge where the peats below approx 30 cm become predominantly minerogenic the modern rates of sc accumulation past 100 years are similar in the two cores 39 g m 2 a 1 at egr versus 52 g m 2 a 1 at tge however comparison with deeper sections of the egr core reveals that the present day rates of atmospheric sc deposition are nearly 3 times greater than the long term average rate reflecting the higher concentrations of soil derived atmospheric aerosols today the rates of sc accumulation in the tge core are slightly higher than at egr this reflects the incorporation of mineral matter from the underlying sediments in the deeper minerogenic peats at tge the long term rates of fe and ca accumulation are 5 and 7 times higher respectively than in the egr core these fluxes greatly exceed the difference in sc flux and cannot be explained by the differences in amount of mineral matter in the peats these enrichments require an independent explanation and are most likely the result of adsorption and or complexation of cationic ca and fe species diffusing from deeper layers in the profile the mineralogical composition of the ash is mainly quartz 60 90 with feldspar 5 15 and muscovite 5 15 although various other minerals 5 10 are commonly present biogenic si represents another important fraction of ash and is abundant 30 70 at discrete depths the vertical profiles showed no significant changes in mineralogy with depth assuming a constant composition of source materials the lack of progressive mineralogical change suggests that the fine grained silicates supplied by soil dust have not been measurably weathered during the past two millennia a number of peaks in fe sc are found in both cores above peaks in ash content and density these appear to correspond with temporal fluctuations in wet dry conditions accumulation of fe oxides during dry periods and leaching during wetter conditions

oxygen isotopic diversity of chondrule precursors and the nebular origin of chondrules feo poor type i porphyritic chondrules formed by incomplete melting of solid dust precursors via a yet elusive mechanism two settings are generally considered for their formation i a nebular setting where primordial solids were melted e g by shock waves propagating through the gas and ii a collisional planetary setting here we report a method combining high current electron microprobe x ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors we find that these olivine crystals are ca al ti poor relative to host olivine crystals their variable 17o even in individual chondrule is inconsistent with derivation from planetary interiors as previously argued from 120 triple junctions also exhibited by the chondrules studied herein this indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions we propose that porphyritic chondrules formed during gas assisted melting of nebular condensates comprising relict olivine grains with varying 17o values and ca al ti rich minerals such as those observed within amoeboid olivine aggregates incomplete melting of chondrule precursors produced ca al ti rich melts cat melts allowing subsequent crystallization of ca al ti rich host olivine crystals via epitaxial growth on relict olivine grains incoming mgo and sio from the gas phase induced i the dilution of cat melts as attested by the positive al ti correlation observed in chondrule olivine crystals and ii buffering of the o isotope compositions of chondrules as recorded by the constant 17o values of host olivine grains the o isotopic compositions of host olivine grains are chondrule specific suggesting that chondrules formed in an array of environments of the protoplanetary disk with different 17o values possibly due to variable solid gas mixing ratios

iron bentonite interactions in the kawasaki bentonite deposit zao area japan abstract greenish veins occurring in brecciated bentonite were found in the kawasaki bentonite deposit of the zao region in miyagi prefecture japan their occurrence possibly indicates the interaction of bentonite with fe rich hydrothermal solutions in order to prove the hypothesis and understand the long term mineralogical and petrographic evolution of bentonite during such interactions the greenish veins and the surrounding altered bentonite were analyzed using x ray fluorescence xrf scanning electron microscopy sem x ray diffraction xrd electron probe micro analysis epma scanning transmission electron microscopy with energy dispersed spectroscopy stem eds and micro x ray absorption near edge structure xanes the greenish veins resulting from hydrothermal solution are composed of mixed layer minerals consisting of smectite and glauconite glaucony pyrite and opal the occurrences indicate that glaucony and pyrite formed almost simultaneously from hydrothermal solution prior to opal precipitation the mineral assemblages of the greenish veins and their surroundings indicate that the hydrothermal activity had most likely taken place at a temperature of less than 100 c and that the ph and eh conditions of the reacted solution were neutral to alkaline ph and reducing the unaltered bentonite is composed mainly of al smectite and opal these minerals coexist as a mixture within the resolution level of the microprobe analyses on the other hand the bentonite in contact with the greenish veins consists of discrete opal grains and dioctahedral al smectite containing fe and was altered mineralogically and petrographically by the hydrothermal activity both the clay minerals and the opal were formed by dissolution and subsequent precipitation from the interaction of the original bentonite with the hydrothermal solution because of the similarity of the alteration conditions to those in the geological disposal environment it was considered that the occurrence of fe bentonite interactions in the kawasaki bentonite deposit could yield valuable input for predicting bentonite stability under disposal conditions

petrology of alh85085 a chondrite with unique characteristics abstract alh85085 has several characteristics which set it apart from any known chondritic group its chondrules are smaller 25 75 m the majority are cryptocrystalline and all are volatile na k s depleted feni metal is more abundant 40 9 wt than in any other chondrite and sulfide abundance is low 1 1 matrix material does not surround the chondrules and fragments but occurs as lithic clasts lumps up to 300 m across the bulk composition of alh85085 is lower in na si and s si and higher in ni si fe si and mg si than in other chondritic groups the ca si al si and mg si ratios are closest to those of the renazzo chondrite grouplet cr matrix lumps are most similar to cr and murchison group cm chondrite matrices survival of this fragile matrix material as lithic clasts indicates that they were added just prior to the agglomeration of the chondrite all components chondrules fragments matrix lumps fall within a narrow size range which may be the result of aerodynamic sorting if so the sorting must have occurred after chondrule solidification and following a period of chondrule breakage since fragments from larger chondrules are present the ni vs co trend in the feni metal overlaps that of metal in cr chondrites and bencubbin these trends are similar to those of calculated nebular condensation paths indicating a nebular origin the cryptocrystalline texture and volatile depletion of the chondrules indicate that most chondrules were heated to temperatures above the liquidus although alh85085 cannot now be assigned to any known chondrite group it shares many similarities with cr chondrites and the bencubbin metal silicate chondritic assemblage and may be part of a chondritic group closely related to these meteorites

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

a magmatic origin for silica rich glass inclusions hosted in porphyritic magnesian olivines in chondrules an experimental study rare silica rich glass inclusions 69 sio2 82 wt are described within magnesian olivines of porphyritic type ia chondrules these glass inclusion compositions are clearly out of equilibrium with their host mg olivines and their presence within the olivines is generally attributed to an unclear secondary process such as a late interaction with nebular gases we performed dynamic crystallisation experiments that demonstrate that these si rich glass inclusions are actually magmatic in origin and were trapped inside olivines that crystallized slowly from a magma with a ci i e solar composition their silica rich compositions are the consequence of the small volumes of inclusions which inhibit the nucleation of secondary crystalline phase ca poor pyroxene but allow olivine to continue to crystallize metastably on the walls of the inclusions we suggest that si rich glass inclusions could be the only reliable relicts of what were the first magmas of the solar system exhibiting a ci i e non fractionated composition

moisture variability in the southwestern united states linked to abrupt glacial climate change the atmospheric response to millennial scale circulation changes in the north atlantic ocean during the last glacial period has been difficult to constrain cave deposits from southwestern north america reveal that atmospheric moisture in this region increased in response to slowdowns of the atlantic meridional overturning circulation

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

lime silicates in amoeboid olivine aggregates in carbonaceous chondrites indicator of nebular and asteroidal processes mno feo ratios in olivine from amoeboid olivine aggregates aoas reflect conditions of nebular condensation and can be used in concert with matrix textures to compare metamorphic conditions in carbonaceous chondrites lime low iron mn enriched olivine was identified in aoas from y 81020 co3 05 kaba cv 3 1 and in y 86009 cv3 y 86751 cv3 nwa 1152 cr cv3 but was not identified in aoas from efremovka cv3 1 3 4 or allende cv 3 6 according to thermodynamic models of nebular condensation lime olivine is stable at lower temperatures than mn poor olivine and at low oxygen fugacities dust enrichment 95 in our study whereas efremovka aoas are slightly fe rich fo 92 aoa olivines from kaba are mostly forsteritic but rare fe rich olivine precipitated from an aqueous fluid a combination of precipitation of fe rich olivine and diffusion of fe into primary olivine grains resulted in iron rich compositions fo97 59 in allende aoas variations from fine grained nonporous matrix toward higher porosity and coarser lath like matrix olivine can be divided into six stages represented by 1 y 81020 efremovka nwa 1152 2 y 86751 lithology b 3 y 86009 4 kaba 5 y 86751 lithology a 6 allende these stages are inferred to represent general degree of metamorphism although the specific roles of thermally driven grain growth and diffusion versus aqueous dissolution and precipitation remain uncertain

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

occurrence and distribution of moganite in agate chalcedony a combined micro raman rietveld and cathodoluminescence study agate chalcedony samples of different origin were investigated by performing raman x ray diffraction using rietveld refinement and cathodoluminescence measurements these analyses were performed to measure the content and spatial distribution of the silica polymorph moganite which is considered to represent periodic brazil law twinning of quartz at the unit cell scale in agate chalcedonies homogeneous standard samples including the nearly quartz free moganite type material from gran canaria were analysed in order to compare results of the x ray diffractometry and raman spectroscopy techniques and to provide a calibration curve for the raman results however due to the different length scales analysed by the two techniques the moganite content in microcrystalline sio2 samples measured by raman spectroscopy short range order was found to be considerably higher than the moganite content measured by x ray diffractometry long range order the difference is explained by the presence of moganite nanocrystals nano range moganite lamellae and single brazil law twin planes that are detected by vibrational spectroscopy but that are not large enough in the sense of coherently scattering lattice domains to be detected by x ray diffractometry high resolution raman analysis provides a measure of the moganite content and its spatial variation in microcrystalline silica samples with a lateral resolution in the m range variations in the moganite to quartz ratio are revealed by varying intensity ratios of the main symmetric stretching bending vibrations a1 modes of quartz 465 cm 1 and moganite 502 cm 1 respectively traces of raman microprobe analyses perpendicular to the rhythmic zoning of agates revealed that the moganite to quartz ratio is often not uniform but shows a cyclic pattern that correlates with the observed cathodoluminescence pattern colour and intensity data obtained from an agate sample from a fluorite deposit near okorusu namibia and from a volcanic agate from los indios cuba were selected for detailed presentation variations of cathodoluminescence and raman data between single bands in agates suggest alternating formation of fine grained highly defective chalcedony intergrown with moganite and coarse grained low defect quartz multiple zones indicate dynamic internal growth during a self organizational crystallization process from silica rich fluids

climate variability over the last two millennia in the north american monsoon region recorded in laminated lake sediments from laguna de juanacatlan mexico high resolution titanium ti data obtained using an itrax xrf core scanner from a laminated sediment core from the laguna de juanacatlan western central mexico yield a unique high resolution record of runoff precipitation change for the last 2000 years in the absence of reliable long term meteorological records comparison of the ti data with information from the rich spanish colonial period archives and the post independence period confirms that ti is a proxy for runoff this interpretation is supported by comparison with other high resolution archives from the surrounding region primarily tree rings and other lake sediment sequences the juanacatlan ti record is therefore a proxy for summer monsoonal rainfall the record provides new evidence from the pacific margin of tropical north america of the occurrence of dry conditions through much of the classic period c ad 300 900 and wetter conditions during the later medieval period c ad 1200 1350 the period commonly known as the little ice

suppression of el nino during the mid holocene by changes in the earth s orbit a number of recent reports have interpreted paleoproxy data to describe the state of the tropical pacific especially changes in the behavior of the el nino southern oscillation enso over the holocene these interpretations are often contradictory especially for the eastern tropical pacific and adjacent areas of south america here we suggest a picture of the mid holocene tropical pacific region which reconciles the data enso variability was present throughout the holocene but underwent a steady increase from the mid holocene to the present in the mid holocene extreme warm el nino events were smaller in amplitude and occurred less frequently about a mean climate state with a cold eastern equatorial pacific and largely arid coastal regions as in the present climate this picture emerges from an experiment in which a simple numerical model of the coupled ocean atmosphere system in the tropical pacific was driven by orbital forcing we suggest that the observed behavior of the tropical pacific climate over the mid to late holocene is largely the response to orbitally driven changes in the seasonal cycle of solar radiation in the tropics which dominates extratropical influences

prolonged wet period in the southwestern united states through the younger dryas the younger dryas was one of the more dramatic climatic transitions ever recorded how these types of climatic shifts are expressed in continental interiors is of primary scientific interest and of vital societal concern here we present a speleothem based absolutely dated record using uranium series data of climate change for the southwestern united states from growth chronology of multiple speleothems the stalagmite growth represents the onset of wetter climate 12 500 yr b p soon after the start of the younger dryas the wetter climate persisted a millennium beyond the termination of the younger dryas this wet cycle is likely related to a more southern positioning of the polar jet stream in response to cooler northern hemisphere climate the end of the wet period coincides with the peak of the holocene summer insolation maximum ca 10 500 yr b p the allerod prior to the younger dryas which corresponds to clovis occupation in the southwestern united states was drier in comparison and seems in line with a climatic contribution to megafauna extinction

large holocene lakes and climate change in the chihuahuan desert lake level variations preserved as beach ridges in the laguna el fresnal and laguna santa maria subbasins northern mexico record millennially spaced episodes of increased precipitation during the holocene epoch we find that the early middle and late holocene were punctuated by periods wet enough to establish large pluvial lakes in currently dry basins in the chihuahuan desert the largest dated pluvial lake covered 5650 km2 during the early holocene constructional beach ridges in these subbasins are 221 33 14c yr b p little ice age equivalent 3815 52 to 4251 59 14c yr b p early neoglacial 6110 80 to 6721 68 14c yr b p mid holocene and 8269 64 to 8456 97 14c yr b p early holocene dates that correlate with other millennially spaced wet or cold events in the northern hemisphere we attribute these wet episodes to increased precipitation cooler temperatures and reduced evaporation following southward shifts in winter storm tracks which are related to long term el nino southern oscillation variability during the holocene

an enhanced role for the tropical pacific on the humid pleistocene holocene transition in southwestern north america abstract climate effects on landscape evolution during the late pleistocene holocene transition 14 6 8 ka in southwestern north america traditionally are linked to the activity of the north american monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity we performed an integrated analysis of regional alluvial fan lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies our focus was on the potential role the tropical pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region our analysis indicates that the onset of alluvial fan aggradation in most of the region at 13 5 ka could have been a response to semi permanent el nino like conditions in the tropical pacific which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region the north american monsoon was restricted in extent and intensity until 7 ka and probably was not a major factor in alluvial fan aggradation a second stage of alluvial fan aggradation from 11 5 to 9 ka was dominated by hyper concentrated flows and sheet flood sedimentation along with deposition in fluvial settings storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra tropical cyclones with moisture advected directly from the tropical pacific at 8 ka favorable conditions for the occurrence of these storms waned and storm tracks shifted northward analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the tropical pacific can affect storm properties enough to explain the observed geomorphic effects regardless of other factors traditionally considered of large impact like vegetation change our results suggest that the tropical pacific plays a larger role than currently thought in landscape evolution of the region

evidence for fractional condensation and reprocessing at high temperatures in ch chondrites we performed a detailed study of silica rich components src in the paired ch chondrites acfer 182 and 207 these srcs appear either as chondrules or fragments and they contribute 0 1 vol to the bulk meteorite they usually contain a silica and a silicate portion both portions are in most cases cryptocrystalline and have bulk sio2 concentrations between 65 and 85 wt the silicate generally has a pyroxene normative composition the silica often appears as blebs within the silicate matrix or vice versa if there are no blebs silica and silicate still form rounded interfaces the srcs are depleted in refractory elements like ca al and ti relative to ci a few src like objects are extremely rich in mn and show no depletion in refractory elements we conducted micro raman studies on the silica portions of the srcs to determine their structure and we identified several silica phases quartz cristobalite glass and a yet unidentified polymorph the silicate portion is glass when the silica is glass and crystalline when the silica is crystalline the low contents of al and ca make an igneous origin of the srcs very unlikely and the absence of metal excludes the formation by reduction of pyroxene we suggest instead a fractional condensation origin of the srcs from a si enriched gas after removal of gaseous mg by forsterite condensation additional evidence for fractional condensation is provided by a unique layered object with olivine in the core pyroxene and metal at the rim and silica at the outermost border these layers record the condensation sequence two chondrules were found with several percent of mn and high cr na and k contents providing further evidence for condensation from a fractionated gas the texture of the srcs and the occurrence of cristobalite and silica glass however require formation by liquid immiscibility at high temperatures above 1968 k and subsequent fast cooling therefore we propose a 2 stage model for the formation of srcs in ch chondrites 1 fractional condensation of forsterite enstatite and sio2 rich phases and 2 reheating of sio2 rich components to temperatures above 1968 k followed by rapid cooling all other phases identified in ch chondrites can be understood within the framework of this model thus the extremely unequilibrated ch chondrites provide a wealth of evidence for fractional condensation processes in the early solar nebula in metals meibom et al 1999 and in silicates

thermochemically induced transformations in al smectites a spanish natural analogue of the bentonite barrier behaviour in a radwaste disposal abstract the thermal effect induced by the morron de mateo volcanic dome cabo de gata volcanic region spain on the adjacent bentonitised tuffaceous beds has been studied as a natural analogue of the thermal behaviour of the bentonite engineered barrier of a geological radwaste repository these bentonites consist mainly of fe rich smectites and were formed in equilibrium with seawater at temperatures between 75 and 95 c according to the 18o and d values in contrast bentonites from other localities in the region consist mainly of al smectites formed in equilibrium with meteoric water below 25 c this investigation is focussed on the detection of the chemical differences between smectites from proximal and distal zones to the dome as well as to test whether the temperatures calculated based on the o and h isotopic values correspond to their formation or transformation the initial hypothesis was that the chosen smectites could be formed under marine conditions being later transformed and isotopically re equilibrated as a result of the intrusion to check this hypothesis a detailed mineralogical chemical geochemical and isotopic study has been performed on the smectitised tuffaceous materials and the overlaying biocalcarenites outcropping near and far from the dome the results show that distal smectites are dioctahedral al smectites similar to those from other deposits in the region while proximal smectites are fe and mg rich smectites showing two evolutionary trends on a fe mg al ternary diagram similar features are observed when their structural formulae are plotted on the muscovite celadonite pyrophylite diagram thus they plot in the smectite domain with interlayer charge less than 1 which is mainly due to octahedral substitution for distal smectites while for proximal ones it is caused by both octahedral and tetrahedral substitutions in this ternary diagram the domains of both proximal and distal smectites are partially overlapped the coexistence of di and trioctahedral smectites was only detected in one proximal sample further proximal biocalcarenites are enriched in fe rich dolomite in relation to the distal ones the 87sr 86sr and 13c values in carbonates and d in smectites indicate equilibrium with seawater in contrast 18o values of carbonates and smectites indicate that they were transformed and re equilibrated between 40 and 90 c and between 55 and 66 c respectively independently of their location with respect to the dome these data suggest that the transformation of calcite into mg fe carbonates and the occurrence of fe and mg rich smectites near to the dome resulted from a chemically induced process at similar temperatures the compositional differences among samples suggests that fe mg and minor mn were supplied by a contaminant plume originated from the dome migrating through the sediments and becoming more diluted away from the source the absence of a well defined thermal gradient in the system could be due to the small size semi closed and shallow character of the basin as well as to its high underlying volcanic activity finally the results are discussed in terms of analogue processes that can be expected in the bentonite barrier of a radwaste geological repository

silicic acid biogeochemistry in the gulf of california insights from sedimentary si isotopes reflects transient iron limitation our new d 30 si record from the guaymas basin shows dramatic variations at millennial timescales low d 30 si values synchronous with heinrich events are interpreted as resulting from the decline in si oh 4 utilization at times of decreased upwelling strength while nearly complete si oh 4 utilization was observed at times of invigorated upwelling and increased opal burial during the holocene the bolling allerod and the last glacial period we attribute the complete utilization of si oh 4 to the occurrence of transient fe limitation at these times our study highlights the importance of fe limitation on si and c cycling in coastal upwelling regions and suggests that upwelling dynamics in combination with fe availability have the potential to modulate marine si distribution and opal burial even at short timescales citation pichevin l r s ganeshram b c reynolds f prahl t f pedersen r thunell and e l mcclymont 2012 silicic acid biogeochemistry in the gulf of california insights from sedimentary si isotopes paleoceanography 27 pa2201

silica bearing chondrules and clasts in ordinary chondrites unusual silica bearing chondrules and clasts have been observed in several unequilibrated ordinary chondrites there appear to be two distinct types i silica low ca pyroxene assemblages and ii silica fayalite intergrowths coexisting with low ca pyroxene only the former types are chondrules the bulk compositions of most of these objects can be expressed in terms of three components mgo feo and sio 2 since ca al and na are essentially trace elements secondary alteration has probably perturbed the fe mg ratios in most objects and produced ca rich pyroxenes in some type ii clasts but the silica low ca pyroxene and fayalite are hard to understand as other than primary phases such silica rich bulk compositions can be understood as an intermediate temperature condensate of a solar gas formed by the loss of refractory solid material by gas solid separation before the complete condensation of silicon alternatively the silicarich bulk compositions could have been formed by extreme reduction of normal chondritic olivine and orthopyroxene however the observed ca al depletion and the lack of ni poor metals are not explained by this model the fayalite bearing clasts can be explained by a relatively complex two stage process involving reduction followed by oxidation partial reduction of olivine in the presence of s produces mg rich pyroxene silica and fefes oxidation of metal and sulfide followed by reaction with silica forms fayalite an alternative is that the fayalite bearing clasts result from the decomposition of ferrosilite but this requires the production of metastable ferrosilite during shock or some other high pressure event the simplest unflawed alternative is that the fayalite bearing clasts result from the mixing of a partially molten silica fayalite liquid with mg rich pyroxene

the l3 6 chondritic regolith breccia northwest africa nwa 869 i petrology chemistry oxygen isotopes and ar ar age determinations northwest africa nwa 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons it is an l3 6 chondrite and probably represents the largest sample of the rare regolith breccias from the l chondrite asteroid it contains unequilibrated and equilibrated chondrite clasts some of which display shock darkening impact melt rocks imrs both clast free and clast poor are strongly depleted in fe ni metal and sulfides an unequilibrated microbreccia two different light inclusions and two different sio2 bearing objects were found although the matrix of this breccia appears partly clastic it is not a simple mixture of fine grained debris formed from the above lithologies but mainly represents an additional specific lithology of low petrologic type we speculate that this material stems from a region of the parent body that was only weakly consolidated one imr clast and one sio2 bearing object show 17o values similar to bulk nwa 869 suggesting that both are related to the host rock in contrast one light inclusion and one imr clast appear to be unrelated to nwa 869 suggesting that the imr clast is contaminated with impactor material 40ar 39ar analyses of a type 4 chondrite clast yield a plateau age of 4402 7 ma which is interpreted to be the result of impact heating other impact events are recorded by an imr clast at 1790 36 ma and a shock darkened clast at 2216 40 ma demonstrating that nwa 869 escaped major reset in the course of the event at approximately 470 ma that affected many l chondrites