preserved flora and organics in impact melt breccias impact cratering can destroy life from local to global scales and result in sudden turnovers of dominant genera and or species here we report that it can also preserve components of the local biology present at the time of impact we have investigated floral matter encapsulated within cenozoic era impact glasses produced by separate bolide impacts into the loessoid sediments of argentina that occurred between 9 2 ma miocene and 6 ka holocene the encapsulation preserved not only macro scale morphological biosignatures such as vascular bundles veins phytoliths and papillae but also structures down to the cellular level in the best preserved samples we also found evidence for organic matter while fossilization typically occurs over an extended time period as minerals slowly replace organic matter and the host rock lithifies under pressure the process documented here is instantaneous preservation of morphological and chemical biosignatures in impact events can provide snapshots of the ecology in environments that do not otherwise promote a diverse fossil record we suggest that this would provide a new strategy for identifying signs of possible early life on ancient mars where similar target conditions once existed

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

scaling and parametrization of clear sky solar radiation over complex topography 1 solar radiation at the land surface is influenced by slope aspect shadows and obstruction of the sky all of which vary over a wide range of length scales in regions of complex topography with important consequences for the surface energy balance atmospheric models however generally assume the surface to be flat on subgrid scales for four areas in north america ranging in latitude from 39 n to 69 n and in topography from rolling to mountainous we simulate spatial patterns of clear sky incoming solar radiation it is found that distributions of slope components and variations in shaded area with solar elevation can be approximated by simple functions that scale to each of the areas studied from these results parametrizations are developed for averages standard deviations and distributions of direct beam and diffuse solar radiation results from these parametrizations and from a modified form of a simpler parametrization presented previously compare well with statistics from the spatial simulations the only topographic input required by the parametrizations is the standard deviation of slope components this parameter is again found to have simple scaling relationships with the resolution and extent of the underlying elevation grid and with the standard deviation of elevation

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

the caviomorph rodents from the san andres formation east central argentina and global late pliocene climatic change the caviomorph rodents from the san andres formation are revisited the fossiliferous stratigraphic unit outcrops in the coastal cliffs of the chapadmalal area east central argentina and represents the late pliocene upper marplatan stage sanandresian substage a new species of cavia caviidae and unpublished materials of dolichotis salinicola caviidae and abrocoma abrocomidae are described and the taxonomic status of the previously described taxa is briefly discussed the living taxa cavia d salinicola and abrocoma are first recorded in the san andres formation and the extinct abalosia octodontidae is exclusive of this formation the specimens of abrocoma represent the single undoubted record of the living genus the record of d salinicola abalosia abrocoma and the octodontid ctenomys represents an immigration event to east central argentina this fauna from san andres is the extinct caviomorph assemblage most clearly indicative of arid environments so far recorded its episodic character and composition and the available palaeomagnetic data reinforce the hypothesis that it is probably coeval with the profound late pliocene cooling and drying pulse detected worldwide around 2 5 ma the sanandresian immigrant taxa would have inhabited the emergent semi deserts of western argentina and may have reached the more eastern chapadmalal area during an expansion of such arid environments triggered by this late pliocene cooling and drying pulse

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

a refined astronomically calibrated 40ar 39ar age for fish canyon sanidine abstract intercalibration between the astronomical and radio isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated multi chronometer geologic timescale here we report a high precision 40 ar 39 ar age for the fish canyon sanidine fcs neutron fluence monitor by multi collector noble gas mass spectrometry through cross calibration with a1 tephra sanidines a1ts of the direct astronomically tuned faneromeni section crete the astronomically intercalibrated 40 ar 39 ar age of fcs of 28 172 0 028 ma 2 external errors is within the uncertainty of but more precise 0 10 than the previous 40 ar 39 ar age determined by intercalibration with astronomically tuned tephras from the melilla basin morocco using this proposed age for fcs combined with measurements using the a1ts as the neutron fluence monitor a weighted mean bishop tuff 40 ar 39 ar sanidine age of 0 7674 0 0022 ma 2 external errors is indistinguishable from the id tims u pb zircon age 0 7671 0 0019 ma the consistency between the astronomically calibrated 40 ar 39 ar sanidine age and u pb zircon age for this quaternary unit suggests that agreement between these two radio isotopic dating techniques is now achievable at better than 0 3 2 in the youngest part of geologic time

the plio pleistocene record of the central eastern pampas buenos aires province argentina the chapadmalal case study abstract the plio pleistocene record of central eastern pampas argentina is preserved in loess like sediments exposed in the sea cliffs south of mar del plata the uplift of the andes during miocene triggered the deposition of wind blown material the source areas are located about 1000 km away in western argentina volcanic ashfalls play a significant role in the formation of eolian sediments which were reworked and redeposited by aqueous transport in a flat low relief landscape under varying climatic conditions vertebrate fossil assemblages document warm and wetter pliocene and early pleistocene climates during montehermosan and uquian land mammal ages more arid and cooler conditions alternating with humid and warmer intervals prevailed during the middle to late pleistocene in ensenadan and lujanian land mammal ages the chapadmalal locality is the best exposed and most complete section almost all the units have been modified by pedogenesis truncated paleosols as well as pedogenic and non pedogenic calcretes are very frequent throughout the stratigraphic succession several paleosurfaces with prominent paleosols and calcretes suggest that sedimentation was an episodic process with numerous depositional gaps recording relatively long lasting intervals of landscape stability the depositional events represent alternations of fluvial and eolian sedimentation low frequency catastrophic episodes played an important role in the sedimentation process

topographic distribution of clear sky radiation over the konza prairie kansas this research analyzes the topographic distribution of clear sky incoming solar radiation over the tallgrass konza prairie site of fife the first islscp international satellite land surface climatology program field experiment using a two stream atmospheric radiation model and digital elevation grids of 25 50 and 100 m grid spacing clear sky radiation is simulated throughout the day for three dates december 15 march 15 and june 15 geostatistical analysis is used to characterize the spatial and temporal variability in modeled radiation at each grid spacing the variance and spatial autocorrelation of simulated incoming radiation depend on sun angle and elevation grid spacing the behavior of the variance as a function of sun angle optical depth and mean terrain slope can be explained by considering direct radiation variability on a simplified terrain model of uniform albedo where slopes are equal and azimuths are distributed uniformly in all directions for this constant slope model it can be shown analytically that the solar zenith angle at which variance is maximized is a function of optical depth only and is independent of elevation slope and aspect results from the two stream simulations support this conclusion and suggest its applicability to real terrain

pleistocene glass in the australian desert the case for an impact origin irregular masses and flat slabs of vesicular slaglike and glassy melt referred to herein as edeowie glass are locally abundant on a desert plain in central south australia where the material appears to be associated with an old land surface being exhumed by deflation and water erosion the slabs of melt are associated with outcrops of baked sediment having very similar geochemistry suggesting an origin by in situ surface fusion embedded clasts displaying shock metamorphism in quartz suggest that the thermal source may have been in some way associated with an impact event although an obvious crater is lacking if edeowie glass is related to impact a different thermal mechanism from that generally ascribed to the production of impact melt is required because of evidence for in situ generation of melt distal from any crater ar 40 ar 39 laser probe dating of two samples has produced overlapping dates of 0 67 0 07 and 0 78 0 33 ma

the volcanic and geochemical development of sao nicolau cape verde islands constraints from field and 40ar 39ar evidence abstract we present 34 new age results from 40ar 39ar incremental heating analyses of groundmass separates from volcanic rocks from sao nicolau cape verde combining the age results with field observations we show that the volcanic activity that formed the island occurred in four separate stages 1 6 2 5 7 ma 2 4 7 2 6 ma 3 1 7 1 ma and 4 8 wt basic to ultrabasic ranging from nephelinites through basanites to picrobasalts evolved rocks range to phonolites during all four volcanic stages predominantly high mgo rocks were erupted with 8 19 wt mgo olivine microprobe data from eight highly porphyritic of these rocks had fo86 88 demonstrating that even crystal enriched rocks represent primitive melts and in general it can be concluded that mantle melts reached the surface after only little modification during much of the development of sao nicolau only during stage 2 magma chambers were established and highly developed magmas erupted at this time the major part of the island was formed the new results constrain the evolution of sao nicolau to have taken place during a later time interval than previously assumed with more ages of only around 50 ka the youngest volcanic rocks found on the island indicate that stage 4 may still be active the cyclicity of sao nicolau volcanism suggests that mantle plume material arrived under sao nicolau in batches separated by nonfertile material

an attempt to constrain the age duration and eruptive history of the karoo flood basalt naude s nek section south africa we have carried out paleomagnetic sampling of a similar to 750 m sequence of the karoo large igneous province naude s nek pass south africa k ar dating cassignol gillot has been performed on four samples from the 650 m upper unit mean age 179 2 1 8 ma and a sample from the lower unit 184 8 2 6 ma a succession of two phases of volcanism is suggested the lower 25 flows 115 m thick have recorded a reversed polarity the next 23 flows 135 m thick are transitional and contribute a detailed record of the van zijl 1962 jurassic reversal the upper 38 flows 500 m thick have normal polarity directional groups dgs of lava flows with quasi identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events altogether 19 dgs and 10 sheet lobes yield a sequence of 29 distinct directions this could correspond to a total eruptive activity shorter than 3000 years less than one per mil of the total duration over which the section was emplaced we obtain a new paleomagnetic pole for south africa at similar to 180 ma lambda 75 2 degrees n phi 276 4 degrees e a 95 5 8 degrees n 19 which is consistent with earlier reports

climatic interpretation of a 1 9 ma environmental magnetic record of loess deposition and soil formation in the central eastern pampas of buenos aires argentina abstract much of what we know about quaternary climate has been learned from sedimentary records from the world s oceans with the exception of the extensive studies of the chinese loess paleosol sequence and more recent studies of long lake records there are few long terrestrial climate records particularly from the southern hemisphere the loess record of argentina provides an important opportunity to further our understanding of climate change from a terrestrial environment but its complexity and discontinuity have led to difficulty in formulating a climatological model of depositional and pedogenic processes in this study we present one of the longest and most continuous loess loessoid records from the central eastern pampas of argentina our age model is based on optically stimulated luminescent dates and a paleomagnetic reversal stratigraphy and indicates a basal age around 1 9 ma within the age model uncertainties we characterize the environmental magnetic properties associated with loess deposition and soil formation with respect to wind patterns moisture availability and temperature major changes in magnetic grain size are linked to a differential northward shift of the subtropical high pressure cell during glacial periods we suggest that coarser finer magnetic grains correspond to weaker stronger glacial periods when the high pressure cell is located in a more southerly northerly position and the source region is more proximal distal to our study area an abrupt increase in the ultrafine grained magnetic material around 0 9 ma is related to an increase in moisture transport from the south atlantic driven by an increase in summer sea surface temperatures at the mid pleistocene transition 1 ma in addition to these grain size variations there is a relative decrease in the amount of goethite compared to hematite beginning around 0 5 ma which has been related to the temperature increase observed after the mid brunhes event 450 ka in the epica ice core temperature record a more detailed comparison to insolation indicates that for portions of the record ferrimagnetic minerals are depleted during periods of low insolation this result suggests that argentine loess deposition and soil formation follows a model more similar to the alaskan loess sequences than the chinese loess sequences although further work is needed to validate the models and mechanisms proposed in this study our record indicates that the mineral magnetic properties of the loess and paleosol deposits record major changes in deposition and soil formation and provide insight into possible mechanisms relating to global and or hemispheric climate change

hirise views enigmatic deposits in the sirenum fossae region of mars hirise images together with other recent orbital data from mars define new characteristics of enigmatic hesperian aged deposits in sirenum fossae that are mostly 100 200 m thick drape kilometers of relief and often display generally low relief surfaces new characteristics of the deposits previously mapped as the electris deposits include local detection of meter scale beds that show truncating relationships a generally light toned nature and a variably blocky weakly indurated appearance boulders shed by erosion of the deposits are readily broken down and contribute little to talus thermal inertia values for the deposits are 200 j m 2 k 1 s 1 2 and they may incorporate hydrated minerals derived from weathering of basalt the deposits do not contain anomalous amounts of water or water ice deflation may dominate degradation of the deposits over time and points to an inventory of fine grained sediment together with constraints imposed by the regional setting on formation processes these newly resolved characteristics are most consistent with an eolian origin as a loess like deposit comprised of redistributed and somewhat altered volcanic ash constituent sediments may be derived from airfall ash deposits in the tharsis region an origin directly related to airfall ash or similar volcanic materials is less probable and emplacement by alluvial fluvial impact lacustrine or relict polar processes is even less likely

effect of volatiles and target lithology on the generation and emplacement of impact crater fill and ejecta deposits on mars impact cratering is an important geological process on mars and the nature of martian impact craters may provide important information as to the volatile content of the martian crust terrestrial impact structures currently provide the only ground truth data as to the role of volatiles and an atmosphere on the impact cratering process recent advancements based on studies of several well preserved terrestrial craters have been made regarding the role and effect of volatiles on the impact cratering process combined field and laboratory studies reveal that impact melting is much more common in volatile rich targets than previously thought so impact melt rocks melt bearing breccias and glasses should be common on mars consideration of the terrestrial impact cratering record suggests that it is the presence or absence of subsurface volatiles and not the presence of an atmosphere that largely controls ejecta emplacement on mars furthermore recent studies at the haughton and ries impact structures reveal that there are two discrete episodes of ejecta deposition during the formation of complex impact craters that provide a mechanism for generating multiple layers of ejecta it is apparent that the relative abundance of volatiles in the near surface region outside a transient cavity and in the target rocks within the transient cavity play a key role in controlling the amount of fluidization of martian ejecta deposits this study shows the value of using terrestrial analogues in addition to observational data from robotic orbiters and landers laboratory experiments and numerical modeling to explore the martian impact cratering record

evidence for the existence of a north hawaiian ridge current abstract two different datasets are examined for information about the existence and characteristics of the north hawaiian ridge current nhrc the pan pacific drifter dataset shows a mean nhrc to be confined to within about 100 km of the northern side of the hawaiian ridge a close look at the distribution of the data shows that the current is mostly confined in time to a two month period in the fall of 1991 mbt data from the trade wind zone oceanography pilot study conducted in the mid 1960s were also examined of the 14 cruises with sections north of the hawaiian ridge 5 showed evidence of northwestward flow along the ridge of those 5 one was associated with northward meandering of the north equatorial current it is thus suggested that the nhrc is highly intermittent there are three related hypotheses for the formation of the nhrc 1 propagation of rossby waves westward into the ridge 2 forking of the north equatorial current and 3 as a western boundary current for the eastern pacific north

slope distributions threshold hillslopes and steady state topography digital elevation models of two steady state mountain ranges the olympic mountains om and oregon coast range ocr are used to examine relationships between slope distributions the development of threshold hillslopes and steady state topography plots of drainage area versus slope for these mountain ranges exhibit substantial scatter that complicates comparison of range form to analytical theories and landscape evolution models contour plots of the density of such data reveal an attractor at the scale of the transition from hillslope processes to channel processes and log bin averaging reveals trends that parallel predictions of steady state erosion laws but with different rate laws for five distinct process domains hillslopes valley heads and colluvial bedrock and alluvial valley segments slope histograms computed for 100 km2 areas defined by a 10 10 km grid throughout the om exhibit approximately normal or exponential distributions in areas of active rock uplift and depositional topography respectively local slope distributions in the ocr also tend to be normally distributed but some are left skewed in areas with gentler slopes mean slopes determined both over the above referenced grid and a 10 km diam moving window are relatively invariant in the core of the om in spite of strong contrasts in bedrock erodibility and gradients in long term rock uplift rates in contrast the mean slopes in the ocr parallel latitudinal gradients in rock uplift rates and bedrock erodibility hence the slope distributions in the om and ocr reflect distinct relationships between development of threshold bedrock and soil mantled hillslopes and steady state topography

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

high frequency controls on alluvial successions an integrated sedimentological and palaeopedological approach to the plio pleistocene of argentina abstract plio pleistocene palaeosol bearing alluvial strata are continuously exposed along the marine cliffs of southeastern buenos aires province argentina the study interval was deposited by a mixed predominantly suspended load fluvial system outcrops are dominated by floodplain siltstones and mudstones that exhibit a cyclic alternation between calcic and vertic palaeosols this cyclicity is represented by meter scale alluvial cycles that show as a general trend a gradual increase in the proportion of channel fill deposits up section cyclic variations in palaeosols facies and architectural elements can be related to allogenic drivers such as climate and eustasy for the lower punta san andres alloformation the stratigraphic arrangement of the recognised palaeosols and the clay mineral distribution indicate that the palaeoclimate was subhumid average mean annual precipitation map 840 mm seasonal and temperate average mean annual temperature mat 7 85 c for at least 1 ma from ca 2 9 ma to ca 1 8 ma late pliocene early pleistocene with several intervals in which conditions became drier and probably colder even though climate was a major control on palaeosol development it is suggested here that it was not the main control on high frequency cycles variations in the orbital parameters more specifically in insolation were interpreted as having controlled the sedimentation and architectural evolution of the lower punta san andres alloformation depositional systems

quaternary mammalian faunas of the pampean region abstract knowledge of the fossil mammals from the pampean region of argentina has greatly increased in recent years to analyze the patterns of change in regional climates and environments through the quaternary the fossil record of land mammals was used two principal factors affected mammalian diversity in the pampean region during the pleistocene glaciations and the climatic environmental and sea level changes associated with them and the large numbers of north american taxa that immigrated to south america from the pliocene the regional climatic conditions changed from being warm and wet to being colder drier and more seasonal during the pleistocene as a consequence of glacial cycles cold and dry conditions were interrupted by relatively short warmer and wetter periods several pulses of expansion and contraction of steppes and subtropical forests are recorded this change in habitat produced the provincialism that has characterized the mammal faunas from the pampean region finally the most notable characteristic of the quaternary mammal fauna of south america is the almost complete extinction of large mammals during the latest pleistocene earliest holocene the evidence presented here is most consistent with a model that explains megafaunal extinction through climatic fluctuations as it appears that only a few species of large mammals survived until the holocene nevertheless a human presence could have accelerated the process of extinction

a 3 3 ma impact in argentina and possible consequences enigmatic glassy materials escorias and red bricklike materials tierras cocidas occur at a restricted stratigraphic level the top of the chapadmalal formation materials from one locality near mar del plata are attributed to a mid pliocene impact event with a radiometric and magnetostratigraphic age of 3 3 million years ago ma an extinction of endemic fauna including the glyptodonts and flightless cariamid birds correlates with the unit containing the impact glasses moreover the age of the glasses is coincident within dating uncertainties with a pulselike change in the oxygen isotope marine record in the atlantic and pacific oceans just before the late pliocene deterioration of the climate

impact melts and glasses on mars 1 dark mobile materials on mars are widely considered to be volcanically derived windblown particulates such a conclusion is consistent with the large volcanic edifices on mars and the assumption that a volatile rich crust would not favor significant impact melt generation several post viking discoveries however provide new perspectives for reconsidering alternative origins not only for some of the mobile dark veneers but also certain materials at the pathfinder and viking landing sites first certain dark materials on mars do not exhibit the expected mafic absorption features in their reflectance spectra second the pathfinder mission revealed unexpected feldspathic compositions of vesicular blocks that lead to a preliminary conclusion that they represent andesites or basaltic andesites rather than basalts third recent discoveries of holocene to miocene impacts in argentine loessoid deposits provide new insights for the nature of melts generated from eolian sediments fourth global dispersal of impact melt products are now recognized in the terrestrial record throughout the cenozoic consequently the generation and preservation of impact glass on mars should be reconsidered possible occurrences include possible tektite like strewnfields downrange from recent oblique impacts dark avalanches emerging from backwasting cliffs of easily eroded unconformable deposits global dispersal of glassy ejecta from hesperian and amazonian craters of chicxulub scale and dark mobile veneers emerging from noachian craters we propose that the mobile dark materials comprising many of the named features of mars as well as some blocks at the pathfinder viking landing sites may represent impact melt products

palaeoecological implications of the mid pliocene faunal turnover in the pampean region argentina abstract known since darwin s time the plio pleistocene succession exposed at the chapadmalal sea cliffs have provided abundant mammal remains even though palaeobiological studies are scarce different authors have identified several assemblages in this stratigraphic interval of homogeneous lithology which records an important mid pliocene faunal turnover dated at ca 3 3 ma this paper examines the palaeoecological characteristics and the regional importance of this faunal turnover 37 of all genera and 53 of all species became extinct between the older chapadmalalan and the younger barrancalobian chronostratigraphic units a level that is actually higher than the percentage of genera extinct at the global scaled cretaceous tertiary extinction the palaeoecological analysis permits to infer that the species rich chapadmalalan fauna seems to have been very balanced from the trophic energetic point of view on the other hand the succeeding and less explored barrancalobian represents a relatively poorer environment highly unbalanced in terms of the herbivore carnivore relationships due mainly to the absence of medium to large mammal and bird carnivores the cause of the mid pliocene faunal turnover is still debatable climatic change has been traditionally related to this faunal turnover stratigraphically an impact event dated at ca 3 3 ma is recorded in the succession this event coincides with a change in the deep sea stable isotopic record reflecting a sudden change in climate and ocean circulation this suggests that the impact might have induced regional faunal extinctions correlation with other areas different from the coast of buenos aires province is needed to establish the geographic scale of the extinction new fossils will provide a more complete and accurate picture of this remarkable faunal turnover

probabilistic description of topographic slope and aspect 1 local topographic features such as slope and aspect play a crucial role in a number of morphological ecological and hydrological processes we propose a simple yet realistic probabilistic description of local slope and aspect as a function of properties of the field of elevation changes we consider different classes of models of elevation changes and obtain the theoretical distribution of slope and aspect we relate the features of the obtained distributions to large scale landscape structures such as regional trends and anisotropy we find that the theoretical distribution of slope is strongly impacted by the parameters used to represent the distribution of elevation changes while large scale features play a secondary role conversely the distribution of aspect is also controlled by regional trends and anisotopy even when they are weak the proposed statistical description of slope and aspect is applied to assess the effects of topographic features on direct solar radiation mean and standard deviation the main control on direct solar radiation is exerted by the partial derivative variance we consider four different landscapes across the continental united states and compare the proposed theoretical description of slope and aspect distributions to the observed histograms

40k 40ar dating of the main deccan large igneous province further evidence of ktb age and short duration most mass extinctions coincide in time with outpourings of continental flood basalts cfb some 20 years ago it was shown courtillot v besse j vandamme d montigny r jaeger j j cappetta h 1986 deccan flood basalts at the cretaceous tertiary boundary earth planet sci lett 80 361 374 courtillot v feraud g maluski h vandamme d moreau m g besse j 1988 deccan flood basalts and the cretaceous tertiary boundary nature 333 843 846 duncan r a pyle d g 1988 rapid eruption of the deccan flood basalts at the cretaceous tertiary boundary nature 333 841 843 that the age of the deccan traps was close to the cretaceous tertiary kt boundary and its duration under 1 myr we have undertaken a new geochronological study using the unconventional 40k 40ar cassignol gillot technique which is particularly well suited to the potassium poor deccan lavas the mean of 4 determinations from the topmost ambenali and mahabaleshwar formations is 64 5 0 6 ma they straddle the c29r c29n reversal boundary for which they provide a new constraint the mean age of 3 determinations from the oldest jawhar formation is 64 8 0 6 ma the difference in age between top and bottom of a 3500 m composite section probably comprising 80 of the total deccan volume is statistically insignificant with the overall mean age being 64 7 0 6 ma n 7 our results are consistent with the most recent 40ar 39ar determinations knight k b renne p r halkett a white n 2003 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps earth planet sci lett 208 85 99 knight k b renne p r baker j waight t white n 2005 reply to 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps discussion by a k baksi earth planet sci lett 239 374 382 confirming that there should be no systematic difference between the two methods when they are used in an optimal way an earlier smaller but significant pulse of volcanism between 68 and 67 ma extending over at least 500 km in latitude in the northern part of the deccan cfb has also been identified after 2 to 3 ma of quiescence the second major phase of volcanism occurred near 65 ma expanding over most of the area covered by the first pulse and another 500 km to the south consistent with drift of india by 300 to 450 km at not vert similar 150 mm yr during the quiescence period new paleontological data from the remote rajahmundry section keller g adatte t gardin s bartolini a bajpai s humler e in prep the cretaceous tertiary boundary in deccan traps of the krishna godavari basin of southeastern india epsl to be submitted suggest that this second pulse can itself be divided into two major pulses one starting in c29r and ending at the kt boundary the second starting in the upper part of c29r and ending within c29n

40ar 39ar geochronology of flood basalts from the kerguelen archipelago southern indian ocean implications for cenozoic eruption rates of the kerguelen plume the 6500 km2 kerguelen archipelago formed on the northern kerguelen plateau nkp 4 105 km2 which is a shallow submarine plateau belonging to the kerguelen large igneous province in the southern indian ocean flood basalts make up 85 of the archipelago and are interpreted as the most recent volcanism 40 ma from the kerguelen hotspot which has erupted basalt for the last 115 million years based on 40ar 39ar incremental heating of acid leached groundmass separates we report isochron ages ranging from 29 26 0 87 ma to 24 53 0 29 ma for 15 basalts from five stratigraphic sections from diverse regions of the archipelago the oldest dated basalt from the archipelago 29 ma is much younger than the 40 ma age of conjunction between the hotspot and the southeast indian ridge basalt eruption seems to have ceased shortly after 24 ma although small volume highly evolved lavas and plutons continued to form in the archipelago the basalt age data suggest an average lava accumulation rate of 1 6 0 9 km my during the oligocene the archipelago s volumetric eruption rate 0 009 km3 yr is lower than estimates made for the cretaceous kerguelen plateau 1 7 km3 yr and the ninetyeast ridge hotspot track 0 18 km3 yr suggesting that the late cenozoic extrusive activity of the kerguelen plume is waning cenozoic volcanism attributed to the kerguelen plume occurs over a diffuse area with quaternary eruptions at heard and mcdonald islands and within the kerguelen archipelago the decreasing eruption rate and areally diffuse volcanism may be explained by the thick lithosphere of the cretaceous kerguelen plateau overriding and insulating the plume however if the undated nkp which underlies the archipelago formed during the cenozoic then the crustal production rate of the plume from 40 ma to the present 0 25 km3 yr would be similar to the crustal production rate 0 23 km3 my previously estimated for the formation of the ninetyeast ridge 82 38 ma

constraints on landscape evolution from slope histograms we analyzed 10 000 topographic slope histograms subsampled from digital elevation models dems of 28 landscapes throughout the continental united states these landscapes are made up of diverse lithologies and have different climatic and tectonic histories but their slope distributions are consistently unimodal and collectively exhibit a systematic change from positively skewed to negatively skewed as mean slope increases this change not only occurs between landscapes but can also occur within them and is recorded in dems of different spatial resolution the transition from positive to negative skewness cannot be accounted for by scaling of relief but requires a fundamental change in landscape morphology process based landscape modeling reproduces the trend and suggests that a change in the dominant hillslope process from creep and or wash to slope failure is responsible for the transition from positive to negative skewness with increasing mean slope this process transition is governed by a dimensionless uplift number that reflects the balance between processes of relief generation and denudation the uplift number provides a basis for understanding the morphologic similarity between landscapes in our analysis illustrating how different combinations of climate tectonics and lithology can lead to equivalent morphologies

the quaternary impact record from the pampas argentina abstract loess like deposits cover much of central argentina and preserve a rich record of impacts since the late miocene the present contribution focuses on two localities containing quaternary impact glasses along the coastal sequences near centinela del mar cdm and from near rio cuarto rc these highly vesicular glasses contain clear evidence for an impact origin including temperatures sufficient to melt most mineral constituents 1700 c and to leave unique quench products such as cristobolite the cdm glasses occur within a relatively narrow horizon just below a marine transgression expressed by a series of coastal paleo dunes and systematic changes in the underlying sediments high resolution 40ar 39ar dating methods yielded an age of 445 21 ka 2 glasses were also recovered from scattered occurrences lower in the section but were dated to 230 40 ka this inconsistency between stratigraphic and radiometric age is most likely related to a nearby outcrop of glass that had been exposed and locally re deposited in coastal lagoons during the last marine transgression at 125 ka sediments containing the original impact glass layer are now missing due to an unconformity perhaps related to subsequent marine transgressions after the impact 410 ka and 340 ka and hiatuses in deposition two different types of impact glasses from rc yield two distinct dates high resolution 40ar 39ar dating of fresher appearing glasses well preserved tachylitic sheen indicates an age of 6 2 ka 2 independent fission track analyses yielded a similar age of 2 3 1 6 ka 2 more weathered glasses however gave significantly older ages of 114 26 ka 2 consequently materials from two separate quaternary impacts have been recovered at rio cuarto the younger glasses are consistent with previously reported carbon dates for materials on the floor of one of the large elongate structures the depths of excavation for the rc and cdm impacts are very different while the rc glasses are largely derived from near surface materials the cdm glasses from the upper level contain added components consistent with miocene marine evaporites at a depth of about 400 500 m e g high cao and p2o5 the cdm glasses also incorporated older loess like sediments from depth based on the geochemistry several ratios of key trace and rare earth elements of sediments of different ages from the miocene to the holocene indicate a systematic compositional change through time such changes calibrate the observed differences in glass composition from their host sediments and further indicate incorporation of materials from depth consequently the argentine loess like sediments preserve evidence for at least four separate quaternary impacts based on foreign components in the glasses the cdm impact very likely produced a crater now buried or eroded once as large as 6 km in diameter the younger rc glasses however are consistent with shallower excavation consistent with an oblique impact

multiple fluvial reworking of impact ejecta a case study from the ries crater southern germany impact ejecta eroded and transported by gravity flows tsunamis or glaciers have been reported from a number of impact structures on earth impact ejecta reworked by fluvial processes however are sparsely mentioned in the literature this suggests that shocked mineral grains and impact glasses are unstable when eroded and transported in a fluvial system as a case study we here present a report of impact ejecta affected by multiple fluvial reworking including rounded quartz grains with planar deformation features and diaplectic quartz and feldspar glass in pebbles of fluvial sandstones from the monheimer hohensande 10 km east of the ries crater in southern germany

wa1ms a 2 61 ga muscovite standard for 40ar 39ar dating abstract the 40ar 39ar dating technique requires the use of neutron fluence monitors standards to allow calculation of the age of a sample precise calibration of these standards is crucial to obtaining accurate ages and decreasing the uncertainties associated with 40ar 39ar dates few fully intercalibrated 40ar 39ar standards with a sufficient total fusion grain to grain reproducibility are currently in use in the argon community for precambrian samples only hb3gr hornblende 1 08 ga yields sufficient grain to grain reproducibility and has an appropriate age for acceptable argon isotopic ratio measurements here we present chemical and intercalibration results for a new 2 61 ga standard wa1ms is a muscovite extracted from an archaean shear zone in the lake johnston greenstone belt western australia in situ trace element analysis by ela icpms revealed consistent k contents subtle zonation and intra grain and grain to grain heterogeneities in rb sr ti and fe but a lack of mineral inclusions wa1ms has been investigated over 3 irradiations ranging from 25 to 40 h in two reactors with several disc positions and three grains sizes and has been calibrated against fcs and ga1550 and hb3gr overall we carried out 48 total fusion and 4 step heating experiments of wa1ms crystals flat age spectra and average f value 40ar 39ark relative standard deviations ranging from of 0 43 to 0 60 p 0 15 0 83 for 47 48 analyses demonstrate the reproducibility of wa1ms and its suitability as a reliable 40ar 39ar standard we calculated r wa1ms fcs 205 59 0 25 r wa1ms ga1550 57 25 0 06 and r wa1ms hb3gr 3 9713 0 014 all with p 0 14 allowing direct comparison between wa1ms and any standards in used in the community provided that they have been calibrated against any of the three standards used in the calibration and regardless of the age adopted for each of these standards the recently revised decay constant values and standard ages proposed by renne et al 2011 yield a weighted mean age of 2614 2 1 5 ma 0 055 p 0 8 1 for wa1ms against the three standards when calibrated solely against fcs in order to minimize correlated errors between fcs vs ga1550 and hb3gr we calculate a preferred age of 2613 0 2 4 ma 0 09 which is recommended when using this standard additionally this corresponds to a 40ar 40k value of 0 3375 0 0057 10 1 for wa1ms which is independent of the value of the decay constant used for calculation and allows recalculation of the age of wa1ms for any preferred set of decay constants and standard ages using the constants proposed by steiger and jager 1977 directly calculated from the three r values yields an age of 2598 2 3 5 ma corresponding to an identical although less precise 40ar 40k value of 0 3375 0 0086 10 1 benefits of this standard include 1 an increased relative precision per crystal analyzed relative to other standards for proterozoic and archaean samples 2 no post irradiation waiting time due to the absence of ca no need to wait for 37arca decay and 3 loading of standard grains that are optically different from the unknown e g pyroxene or hornblende to avoid standard unknown mixing wa1ms is freely available to the 40ar 39ar community

rossby wave induced secondary flows near barriers with application to the hawaiian ridge abstract we restudy the reflection of baroclinic rossby waves from a nonzontal barrier we add lateral friction to an earlier study by mysak and magaard to obtain a more realistic secondary flow near the barrier contrary to the earlier study we not only calculate the eulerian mean flow but also the stokes drift associated with the incident and reflected waves we apply our theory to the hawaiian ridge where the incidence wave field is known compared to the case of vanishing lateral friction the eulerian mean flow now shows smaller current speeds and wider current bands the narrow eastward jet in the immediate neighborhood of the ridge is now replaced by a much weaker broader eastward flow the subsequent western boundary current and eastward countercurrent are now shifted farther away from the ridge in better agreement with white s analysis of historical temperature data the lagrangian mean flow is stronger than the eulerian mean flow within the first 100 km off the ridge the two flows are mostly

recent grazing impacts on the earth recorded in the rio cuarto crater field argentina the most probable angle of a meteoroid impact on a planet is 45 and an impact at 15 from the horizontal or lower is as likely as at 75 or higher 1 2 yet little direct evidence for oblique impacts exists on the earth for two reasons unless the impact angle is very low any asymmetry created during the initial transfer of energy from impactor to target is lost as the crater is formed 3 moreover the shallow craters formed by oblique impact are more easily obscured by subsequent erosion during routine flights two years ago however one of us r e l noticed an anomalous alignment of oblong rimmed depressions 4 km x 1 km on the otherwise featureless farmland of the pampas in argentina we argue here from sample analysis and by analogy with laboratory experiments that these structures resulted from a low angle impact and ricochet of a chondritic body originally 150 300 m in diameter

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

climatic effects of an impact induced equatorial debris ring 1 several theoretical and laboratory studies suggest that some large impact events are capable of inserting material into space depending on mechanics of the impact this material would quickly coalesce to form a temporary debris ring in orbit around the equator which would cast its shadow on the winter hemisphere the results of an atmospheric general circulation model gcm simulation where an orbiting equatorial debris ring is applied as a boundary condition to the model show how the longer term effects of a major impact could affect the climate system the primary effect is a severe cooling in the tropics and the subtropics especially under the seasonally migrating ring shadow the globe cools and becomes drier with the exception of monsoonal regions that become wetter the hadley cell is weakened resulting in drier tropics and weaker subtropical high pressure cells in the winter hemisphere because the tropics cool more than middle latitude regions the equator to pole temperature gradient becomes shallower resulting in weaker tropospheric winds and less high latitude storminess we suggest that the late eocene impact s 35 5 ma could have generated a geologically temporary orbiting debris ring based on the global distribution of tektites associated with these events and patterns of climate change immediately above the iridium microtektite layer the cretaceous tertiary boundary event while larger did not produce a debris ring we also suggest that an opaque debris ring could have acted as the trigger to at least one episode of global glaciation during the neoproterozoic

late miocene early pliocene stratigraphy and paleoceanography of the south atlantic and southwest pacific oceans a synthesis the stratigraphy and paleoceanography of the late miocene and early pliocene have been examined at six sites in the south atlantic and southwest pacific oceans deep sea drilling project dsdp sites 284 516a 519 588 and 590 and two piston cores from chain cruise 115 a consistent stratigraphy was developed among sites using graphic correlation which resulted in age models for all sites that are tied to the revised paleomagnetic time scale of berggren et al 1985 applying these chronologies we assessed latitudinal and interocean contrasts in the stratigraphic ranges of late miocene early pliocene planktonic foraminiferal and nanno fossil datums salient stratigraphic results include 1 the last appearance datum lad of globoquadrina dehiscens is a late miocene 6 4 ma event in the subtropics and is not useful for the placement of the miocene pliocene m p boundary in this biogeographic province 2 the first appearance datum fad of globorotalia crassaformis occurred at 5 1 ma in the south atlantic near the m p boundary suggesting that gr crassaformis may have first evolved in the south atlantic and later migrated to other regions 3 in the southwest pacific the fads of gr margaritae 5 97 ma gr puncticulata 5 09 ma and gr crassaformis 4 87 ma are significantly time transgressive between temperate and warm subtropical regions time lags of 1 0 m y were required for these species to adapt to physical and or biotic conditions peripheral to their endemic biogeographic provinces 4 between the subtropics of the south atlantic and southwest pacific many planktonic foraminiferal datums fad of dentogloboquadrina altispira gr cibaoensis gr conomiozea gr margaritae and gq dehiscens and lad of gr cibaoensis markedly depart from the correlation suggested by magnetostratigraphy indicating that these datum levels are unreliable for correlation between these ocean basins 5 in contrast available calcareous nannofossil datum levels fall on or near the paleomagnetic correlation line indicating synchroneity of events within the subtropics 6 biostratigraphic magnetic and 87sr 86sr correlation between sites 588 and 519 and the m p neostratotype at capo rossello sicily suggests that the base of the zanclean stratotype occurs at 5 1 5 0 ma in the lower reversed subchron of the gilbert about 2 3 105 years above the gilbert chron 5 boundary oxygen isotopic results from dsdp sites 284 519 and ch115 piston cores confirm a prolonged benthic 18o increase in the latest miocene between 5 6 and 5 0 ma as originally proposed by shackleton and kennett 1975 at dsdp site 588 the benthic 18o record in the latest miocene is marked by high frequency fluctuations with amplitude variations of 0 5 and a long period wavelength component of 400 000 years maximum 18o values however occurred during the late miocene kapitean stage between 5 5 and 5 1 ma the late miocene 18o changes resulted from mid and high latitude cooling and pulses of ice sheet expansion and contraction glacial events were most intense during the latest miocene kapitean stage and occurred at 5 50 5 35 ma and at 5 10 ma glacial events are estimated to have lowered sea level by 40 to 60 m and contributed to the isolation and desiccation of the mediterranean basin during the late messinian interglacial conditions prevailed at 5 2 ma and between 5 0 and 4 1 ma ihe early pliocene the beginning of the pliocene was marked by changes in many proxy climatic indicators at all sites suggesting a prolonged interval of warm interglacial conditions between 5 0 and 4 1 ma during the earliest pliocene supplementary tables are available with entire article onmicrofiche order from american geophysical union 2000florida avenue n w washington dc 20009 documentp86 001 2 50 payment must accompany order

currents observed north of oahu during the first five years of hot abstract shipboard adcp sections from 30 hawaii ocean time series cruises over a 5 year period show a mean flow 15 degrees north of west reaching a maximum of 17 cm s 1 about 50 km north of oahu this north hawaiian ridge current is highly variable from cruise to cruise and from year to year it was generally strong in the sections 1988 1991 and weak or absent in 1992 and 1993 there is no prominent annual cycle unresolved short period oscillations are so energetic as to obscure the mean current in individual sections the 5 year mean is highly significant however at five times the estimated standard error currents also were measured with a shipboard adcp during the nominally 3 day occupations of the hot station aloha the mean over 32 cruises was near 6 cm s 1 in magnitude at all depths from 20 to 200 m but the median was near zero hence the northern boundary of the north hawaiian ridge current was usually south of aloha the inter cruise standard deviation of velocity varied from 22 cm s 1 at 20 m to 11 cm s 1 at 200 m for the zonal component and 13 cm s 1 to 8 cm s 1 for the meridional component intra cruise variability during the 17 cruises with the best sampling in depth and time was dominated by a linear trend near inertial oscillations and diurnal and semidiurnal tides each of these four elements had an rms value near 6 cm s 1 at 20 m decreasing to 2 4 cm s 1 at 250 m the vertical shear field was dominated by the diurnal tide and near inertial oscillations the semidiurnal tide and the trend had larger vertical scales of variability and consequently made smaller contributions to the shear

on the possibility of a ridge current along the hawaiian islands four evenly spaced aircraft deployed expendable bathythermograph and concurrent satellite advanced very high resolution radiometer avhrr surveys were made over an extended area during a 1 year period to observe possible ridge currents along the hawaiian islands predicted or hypothesized by other investigators none of the surveys found extended currents running along the ridge instead fields of energetic eddies were found and a curious cusp shaped sea surface temperature pattern was observed in two of the avhrr fields apparent cyclonic currents around the island of hawaii were also observed we conclude that either ridge currents do not exist or that they exist but are intermittent episodic in nature or exist in a form more complicated than could be observed by this study

loess of southern south america abstract loess and loessoid loess like reworked loess sediments extend across the chaco pampean plains and the nw mountain environments of argentina and in neighbouring countries paraguay brazil uruguay bolivia loessoid sediments are much more abundant than primary loess the beginning of the loessoid sedimentation cycle was related to a phase of late miocene orogeny in the andes the plio pleistocene record is mostly composed of loessoid sediments modified by pedogenesis which produced welded palaeosols the late pleistocene holocene loess record reveals a heterogeneous composition across the region coarse textures and an andean derived volcanic composition prevail in the southern pampas finer textures and material coming from the andes and the other two sources sierras pampeanas and the parana basin characterize the loess deposits of the northern pampas and the eastern chaco a southern patagonian source is also suggested for the mountain valley loess of tucuman although a western andean provenance has been proposed it is believed that the material in the western chaco was derived from the bolivian andes loess deposition was related to a multistage transport mechanism involving fluvial and aeolian processes inferred westerly and southwesterly wind directions as dominant carriers of the aeolian deposits are in agreement with westerly palaeowind simulations using climate models however the role played by westerly tropospheric winds and northerly winds remains to be established

covariance patterns of foraminiferal dgr18o an evaluation of pliocene ice volume changes near 3 2 million years ago oxygen isotope data for a pliocene interval from 3 6 to 2 8 million years ago show a mean increase 0 5 per mil of benthic delta 18 o at about 3 2 million years ago whereas planktic delta 18 o does not increase this lack of covariance indicates that the event at 3 2 million years did not result in a permanent increase in the ice budget of either the northern or the southern hemisphere

groundwater characteristics within loessic deposits the coastal springs of los acantilados mar del plata argentina the urban growth of the southern neighborhoods of mar del plata city provoked significant changes in the groundwater balance of the loessic sequences these regional loessic levels with a significant portion of volcanic ash layers were reported subject to fluoride and nitrate concentrations residential houses pump from sands located 70 m depth and withdraw the sewages to depths less than 5 m these effects cause significant local and seasonal summer increments of the water table outcropping via springs at certain unconformities of the coastal cliffs a mathematical model was applied to analyze the water level lowering at the productive levels while there is a decrease in the quality of the upper levels subject to waste discharges much of this groundwater flow is concentrated in unconformities between different types of sediments human activities have affected the aquifer dynamics increasing the groundwater pumping rates and the return velocity of the sewages this should be considered in the management strategies of coastal hydric resources

ground truth for oblique impact processes new insight from the rio cuarto argentina crater field new evidence for an impact origin of oblong rimmed depressions near rio cuarto cordoba province argentina includes shocked silicate phases e g diaplectic glass thermal decomposition of high temperature mineral clasts e g baddeleyite from zircon rapid quenching very low water contents 0 1 wt and generation of identical glasses in hypervelocity laboratory impact experiments the results indicate that glasses with a wide range in major element concentrations can form from a single target type in a relatively small impact event impact glasses with the greatest volatile loss typically exhibit the greatest meteoritic contamination as defined by cr ni and ir abundances the different impact glass types and the different degrees of impactor contamination are proposed to reflect proximity to the projectile target interface during shallow penetration in an oblique impact consistent with laboratory simulations and planetary analogues

saline intrusions in the intermediate waters north of oahu hawaii abstract the hawaii ocean time series hot hydrographic observations at station aloha 22 45 n 158 w reveal that the region is sometimes the site of intrusive interleaving of different water types near the salinity minimum of the north pacific intermediate waters this interleaving is associated with saline laminae or lenses 5 30 km long submesoscale and 5 100 m thick finestructure the temporal variability of these intermediate waters is investigated statistically and a background water type is defined against which to measure the strength of intrusions the salinity minimum shows a relative maximum in variability caused mainly by the episodic appearance of the saline finestructure intrusions which are correlated with low oxygen and silicate concentrations the intermediate salinity minimum has its southern boundary near the latitude of the hawaiian islands where salinity increases towards the south and oxygen gradually decreases thus stirring processes local to the hawaiian ridge may explain the observations possible mechanisms are suggested including fluctuations of the subtropical gyre southern boundary and a hydrodynamically unstable flow at intermediate depths along the ridge either process could transport relatively saline water into the aloha region and would lead to interleaving driven by double diffusion on scales of the order of 10 m in the vertical

power law scaling of topographic depressions and their hydrologic connectivity topographic depressions areas of no lateral surface flow are ubiquitous characteristics of the land surface that control many ecosystem and biogeochemical processes high density of depressions increases the surface storage capacity whereas lower depression density increases runoff thus influencing soil moisture states hydrologic connectivity and the climate soil vegetation interactions with the widespread availability of high resolution lidar based digital elevation model ldem data it is now possible to identify and characterize the structure of the spatial distribution of topographic depressions for incorporation in ecohydrologic and biogeochemical studies here we use ldem data to document the prevalence and patterns of topographic depressions across five different landscapes in the united states and quantitatively characterize the probability distribution of attributes such as surface area storage volume and the distance to the nearest neighbor through the use of a depression identification algorithm we show that these probability distributions of attributes follow scaling laws indicative of a structure in which a large fraction of land surface areas can consist of high number of topographic depressions of all sizes and can account for 4 to 21 mm of depression storage this implies that the impacts of small scale topographic depressions in the landscapes on the redistribution of material fluxes evaporation and hydrologic connectivity are quite significant

paleogene high elevations in the qiangtang terrane central tibetan plateau abstract the timing of uplift of the tibetan plateau remains controversial with estimates varying from eocene to more recent than pliocene in particular the paleoaltimetry of the qiangtang terrane on the central tibetan plateau is completely unknown here we present new stable isotope results of fluvial lacustrine carbonate cement pedogenic carbonate and marl from the kangtuo and suonahu formations deposited between 51 and 28 ma in the heihuling bamaoqiongzong area of the northern qiangtang terrane the lithofacies associations indicate that the kangtuo formation was deposited in alluvial fan and fluvial floodplain environments and the suonahu formation was deposited in near shore lacustrine playa lake and channelized fluvial environments carbon and oxygen isotope values coupled with the sedimentary facies interpretations point to evaporation and low respiration rates in the eocene oligocene paleosols suggesting an arid climate in the high qiangtang area at the time the 18opsw values of paleo surface water reconstructed from the lowest i e the least evaporated 18oc pdb values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin the paleoelevation of the northern qiangtang terrane is reconstructed as above 5000 m by at least the middle oligocene 28 ma similar to the present elevation in this area the aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high lhasa terrane established by the middle oligocene blocked the northward transport of tropical moisture maintenance of high elevation 5000 m from at least the oligocene to the present suggests that the qiangtang crust was already thickened by that time and that underthrusting of india beneath asia since then has continued to provide additional material while at the same time driving asian lower crust eastward by crustal flow

shortwave radiation parameterization scheme for subgrid topography 1 topography is well known to alter the shortwave radiation balance at the surface a detailed radiation balance is therefore required in mountainous terrain in order to maintain the computational performance of large scale models while at the same time increasing grid resolutions subgrid parameterizations are gaining more importance a complete radiation parameterization scheme for subgrid topography accounting for shading limited sky view and terrain reflections is presented each radiative flux is parameterized individually as a function of sky view factor slope and sun elevation angle and albedo we validated the parameterization with domain averaged values computed from a distributed radiation model which includes a detailed shortwave radiation balance furthermore we quantify the individual topographic impacts on the shortwave radiation balance rather than using a limited set of real topographies we used a large ensemble of simulated topographies with a wide range of typical terrain characteristics to study all topographic influences on the radiation balance to this end slopes and partial derivatives of seven real topographies from switzerland and the united states were analyzed and gaussian statistics were found to best approximate real topographies parameterized direct beam radiation presented previously compared well with modeled values over the entire range of slope angles the approximation of multiple anisotropic terrain reflections with single isotropic terrain reflections was confirmed as long as domain averaged values are considered the validation of all parameterized radiative fluxes showed that it is indeed not necessary to compute subgrid fluxes in order to account for all topographic influences in large grid sizes

the record of miocene impacts in the argentine pampas argentine pampean sediments represent a nearly continuous record of deposition since the late miocene 10 ma previous studies described five localized concentrations of vesicular impact glasses from the holocene to late pliocene two more occurrences from the late miocene are reported here one near chasico ch with an 40ar 39ar age of 9 24 0 09 ma and the other near baha blanca bb with an age of 5 28 0 04 ma in contrast with andesitic and dacitic impact glasses from other localities in the pampas the ch and bb glasses are more mafic they also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals in addition to evidence for extreme heating 1700 c shock features are observed e g planar deformation features pdfs and diaplectic quartz and feldspar in impact glasses from both deposits geochemical analyses reveal unusually high levels of ba 7700 ppm in some samples which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth these two new impact glass occurrences raise to seven the number of late cenozoic impacts for which there is evidence preserved in the pampean sediments this seemingly high number of significant impacts over a 10 6 km 2 area in a time span of 10 myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption the pampean record suggests therefore that either atmospheric entry models need to be reconsidered or that the earth has received an enhanced flux of impactors during portions of the late cenozoic evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous low energy deposition such as the pampas additionally the rare earth element ree concentrations of the target sediments and impact melts associated with the chasic event resemble the hna k australites of similar age this suggests the possibility that those enigmatic tektites could have originated as high angle distal ejecta from an impact in argentina thereby accounting for their rarity and notable chemical and physical differences from other australasian impact glasses

palaeomagnetism of late cenozoic fossiliferous sediments from barranca de los lobos buenos aires province argentina the magnetic age of the south american land mammal ages abstract palaeomagnetic data for a sequence of late cenozoic sediments exposed at bajada martinez de hoz and el muelle barranca de los lobos atlantic coastal area buenos aires province are reported the sequence comprises sediments assigned to the chapadmalal formation montehermosense land mammal age barranca de los lobos vorohue and san andres formations uquiense land mammal age miramar formation ensenadense land mammal age and arroyo seco and arroyo loberia formations lujanense land mammal age the magnetostratigraphy recorded in the sequence suggests a brunhes magnetic age 3 15 ma for the chapadmalal formation it also suggests a brunhes to late matuyama magnetic age for the lujanense land mammal age a matuyama magnetic age for the ensenadense land mammal age a gauss magnetic age for the uquiense land mammal age and an early gauss to gilbert magnetic age for the montehermosense land mammal age finally the results obtained suggest that the panama isthmus was formed at an age earlier than 3 0 ma possibly 4 0 3 5 ma ago

magnetostratigraphy of neogene andean foreland basin strata lat 33 s mendoza province argentina neogene nonmarine strata in the southernmost part of the precordillera thrust belt of the andes of argentina accumulated during the miocene and pliocene in response to eastward advance of thrust sheets located to the west to link the sequence of deformational events with the sedimentary record five stratigraphic sections in the mendoza province were dated using magnetic polarity stratigraphy calibrated with 40 ar 39 ar dates from interbedded tephras these data establish age control on tectonic activity volcanism and deposition the existence of a backarc foreland basin that developed prior to or ca 16 ma is recorded by the oldest strata the ca 15 7 12 2 ma baksi time scale marino formation deposition of this unit is linked to a major phase of thrust activity in the central part of the aconcagua fold and thrust belt in the principal cordillera initiation of deposition coincides with an eastward shift of the subduction related magmatic foci to the argentine principal cordillera the overlying ca 11 7 9 0 ma la pilona formation is thought to record the initiation of exhumation of the frontal cordillera to the northwest of the study region an extensive ash rich unit the 8 9 locally 9 5 to 8 7 ma tobas angostura formation is correlated with the latest and probably the largest extrusive episode in the principal cordillera chronologic data constrain the timing of oil generation in the cuyo basin and development of prospective closures the new data also require that land mammalian fossils found in the rio de los pozos formation are late miocene rather than pliocene as previously believed

the importance of being cratered the new role of meteorite impact as a normal geological process this paper is a personal and in many ways incomplete view of the past development of impact geology and of the newly recognized importance of impact events in terrestrial geological history it also identifies some exciting scientific challenges for future investigators to determine the full range of impact effects preserved on the earth to apply the knowledge obtained from impact phenomena to more general geological problems and to continue the merger of the once exotic field of impact geology with mainstream geosciences since the recognition of an impact event at the cretaceous tertiary k t boundary much current activity in impact geology has been promoted by traditionally trained geoscientists who have unexpectedly encountered impact effects in the course of their work their studies have involved 1 the recognition of additional major impact effects in the geological record the chesapeake bay crater the alamo breccia and multiple layers of impact spherules in precambrian rocks and 2 the use of impact structures as laboratories to study general geological processes e g igneous petrogenesis at sudbury canada and archean crustal evolution at vredefort south africa other research areas in which impact studies could contribute to major geoscience problems in the future include 1 comparative studies between low level 7 gpa shock deformation of quartz and the production of quartz cleavage in both impact and tectonic environments and 2 the nature origin and significance of bulk organic carbon kerogen and other carbon species in some impact structures gardnos norway and sudbury canada

integrated stratigraphy and astronomical tuning of the serravallian and lower tortonian at monte dei corvi middle upper miocene northern italy an integrated stratigraphy calcareous plankton biostratigraphy magnetostratigraphy and cyclostratigraphy is presented for the serravallian and lower tortonian part middle upper miocene of the monte dei corvi section located in northern italy the detailed biostratigraphic analysis showed that both the discoaster kugleri acme and the first influx of neogloboquadrina acostaensis are recorded at monte dei corvi these events which passed unobserved in previous studies play an important role in delineating the serravallian tortonian boundary thermal and alternating field demagnetization revealed a characteristic low temperature component marked by dual polarities the resultant magnetostratigraphy for the upper part of the section can be unambiguously calibrated to the gpts ranging from c5n 2n up to c4r 2r unfortunately the lower part of the section including the serravallian tortonian boundary interval did not produce a reliable magnetostratigraphy despite the fact that some short reversed intervals and a single normal interval are recorded using sedimentary cycle patterns in combination with the calcareous plankton biostratigraphy the section can be correlated cyclostratigraphically in detail to the partially overlapping and previously tuned section of monte gibliscemi on sicily the monte dei corvi section is dated astronomically by calibrating the basic small scale sedimentary cycles to the precession and 65 n lat summer insolation time series of the la93 solution following an initial tuning of larger scale cycles to eccentricity an almost perfect fit is found between the cycle patterns and intricate details especially precession obliquity interference in the insolation target between 8 5 and 10 ma the tuning to precession remains robust for most intervals back to the base of the section dated at 13 4 ma and shows that the section is continuous apart from a possible short hiatus in the tortonian it provides accurate astronomical ages for all sedimentary cycles calcareous plankton events polarity reversals and ash layers and marks a significant improvement of the recently proposed astronomical calibrations of the monte dei corvi section and of parallel sections in the mediterranean astronomical ages for the ancona and respighi ashbeds are significantly older than previously reported 40 ar 39 ar biotite ages even if the revised older age for the fct san dating standard of 28 02 ma is applied the astronomical dating of the magnetic reversals in the monte dei corvi section results in the completion of the astronomical polarity time scale for the last 13 myr the monte dei corvi section has recently been proposed as the stratotype section for the serravallian tortonian boundary despite the moderate to

high resolution sequence stratigraphy and continental environmental evolution an example from east central argentina abstract the aims of this contribution is to establish a high resolution sequence stratigraphic scheme for the continental deposits that constitute the punta san andres alloformation plio pleistocene in east central argentina to analyze the basin fill evolution and to identify and assess the role that extrinsic factors such as climate and sea level oscillations played during evolution of the unit for the high resolution sequence stratigraphical study of the punta san andres alloformation high and low accommodation system tracts were defined mainly on the basis of the architectural elements present in the succession also taking into account the relative degree of channel and floodplain deposits discontinuities and the nature of depositional systems generated during variations in accommodation helped identify two fourth order high accommodation system tracts and two fourth order low accommodation system tracts at a third order scale the punta san andres alloformation may be interpreted as the progradation of continental depositional systems characterized by a braided system in the proximal areas and a low sinuosity single channel system in the distal areas defined by a high rate of sediment supply and discharge peaks which periodically flooded the plains and generated high aggradation rates during the late pliocene and lower pleistocene

influence of topography on some vegetation cover properties abstract we study the influence of nine local non local and combined quantitative topographic characteristics on vegetation cover altitude and density the objects under study are four areas of the rudny altai differentiation of plant properties is shown to depend on relief parameters which control migration and accumulation of water in landscape by gravity these are landsurface curvatures catchment area topographic and stream power indexes in some cases the phytocoenosis characteristics can have higher correlation with non local and combined topographic variables than with local ones this derives from the fact that non local and combined relief attributes take into account a relative location of an area in a landscape so they can better determine topographic prerequisites of substance motion we demonstrate the advisability of the use of digital models and maps of indicated topographic characteristics in plant investigations