changes of mammal assemblages in the pampean region argentina and their relation with the plio pleistocene boundary abstract a gradual impoverishment of autochthonous mammals in the southeastern pampean region central eastern argentina during the late cenozoic is detected it is partially coeval to the gradual immigration of holarctic mammals and to important climatic changes we propose that the plio pleistocene boundary at around 2 5 ma in the area is located between the vorohue formations and the base of miramar formations

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and represents the late pliocene upper marplatan stage sanandresian substage a new species of cavia caviidae and unpublished materials of dolichotis salinicola caviidae and abrocoma abrocomidae are described and the taxonomic status of the previously described taxa is briefly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

glaciations in patagonia and tierra del fuego during the ensenadan stage age early pleistocene earliest middle pleistocene while the ensenadan stage age ca 2 1 to 0 5 ma was characterized by certain mammal associations in the pampean plains the andean ranges of patagonia and tierra del fuego their piedmont areas and in some cases the adjacent lowlands were undergoing a well defined alternating sequence of glacial and interglacial stages driven by the regional climate of southernmost south america and antarctica this paper reviews the available geographical geological and chronological evidence of glaciation during the ensenadan stage age with perhaps more than 15 distinct glacial events including the great patagonian glaciation gpg and other cold episodes that took place before and after it finally a tentative correlation of patagonian glacial episodes and the pampean biostratigraphic stages based on radiometric dating and paleomagnetic studies is herein presented

the polonez cove formation of king george island antarctica stratigraphy facies and implications for mid cenozoic cryosphere development the middle to late oligocene polonez cove formation exposed on south eastern king george island south shetland islands provides rare evidence of mid cenozoic west antarctic cryosphere evolution a revised lithostratigraphy and facies analysis and a review of the palaeoenvironmental significance of the formation are presented here the diamictite dominated basal member of the formation krakowiak glacier member records the presence and retreat of marine based ice on a shallow continental shelf five overlying members are recognized these consist of basaltic sourced sedimentary rocks and lavas and represent a variety of shoreface and shallow continental shelf environments in an active volcanic setting these units contain diverse reworked and ice rafted exotic clasts that become sparse towards the top of the formation suggesting a continuing but waning glacial influence new 40ar 39ar dates from interbedded lava flows indicate a late oligocene age 25 6 27 2 ma for the polonez cove formation but are slightly younger than skeletal carbonate sr isotope ages obtained previously 28 5 29 8 ma there is evidence for wet based subice conditions at the base of the polonez cove formation but no sedimentary facies to suggest substantial meltwater this may reflect a subpolar setting or may result from lack of preservation or a high energy depositional environment a northern antarctic peninsula south shetland islands provenance is probable for most non basaltic clasts but certain lithologies with possible origins in the transantarctic and ellsworth mountains also occur sparsely throughout the formation there is evidence to suggest that the presence of such far travelled clasts within subglacially deposited facies at the base of the formation reflects the advance of a local ice cap across marine sediments containing the clasts as ice rafted material the presence of these clasts suggests that extensive marine based ice drained into the southern weddell sea region and that a strong weddell sea surface current operated both before and during deposition of the polonez cove formation

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

isotope geochemistry of the miocene and quaternary carbonate rocks in rabigh area red sea coast saudi arabia abstract the rabigh area a coastal region north of jeddah city saudi arabia contains raised quaternary coral reefal terraces and reworked coral fragments mixed with sand and gravel this area has a thin exposure lower miocene shallow marine carbonate rocks that laterally pass into evaporites the miocene carbonate and evaporite rocks conformably overly the lower miocene siliciclastic sequence are in turn capped by the harrat basaltic boulders the miocene carbonates are made up of dolomitic packstone wackestone and mudstone whereas the overlying quaternary reefal terraces are composed of coral boundstone and grainstones the quaternary reefal terraces of rabigh area have been dated using the uranium series dating method to obtain precise dates for these corals the calculated ages 128 212 and 235 ka indicate that deposition took place during high sea level stands associated with interglacial times during oxygen isotope stages ois 5 and 7 the youngest age 128 ka clearly corresponds to stage 5e of the last interglacial period the obtained ages correlate well with those of the emerged reefs on the sudanese and egyptian coasts at the western side of the red sea the broad distribution of wet climate pluvial deposits on the continents and high sea level stands indicate a wide geographical range of the interglacial events of the oxygen isotope stages ois 5 and 7 the oxygen and carbon isotopic composition of the miocene and quaternary carbonate rocks in rabigh area show a broad range of 13c and 18o the quaternary carbonate rocks have significantly higher 13c than the miocene ones but low 13c values of the miocene samples likely indicate a high contribution of carbon from organic sources at the time of deposition linear trends are evident in both groups of samples supporting the likelihood of secondary alteration

a quantitative model of magnetic enhancement in loessic soils we present a quantitative model for the climatic dependence of magnetic enhancement in loessic soils the model is based on the widely accepted hypothesis that ultrafine magnetite precipitates during alternating wetting and drying cycles in the soil micropores the rate at which this occurs depends on the frequency of drying wetting cycles and on the average moisture of the soil both parameters are estimated using a statistical model for the soil water balance that depends on frequency and intensity of rainfall events and on water loss by evapotranspiration monthly climatic tables are used to calculate the average soil moisture and the rate of pedogenic magnetite production which is proportional to a new parameter called magnetite enhancement proxy mep our model is tested by comparing mep calculated for known present day climates with the magnetic enhancement of modern soils the magnetic enhancement factor defined as the ratio between a given magnetic enhancement parameter and mep is expected to be a site independent constant we show that magnetic enhancement differences between soils from the chinese loess plateau and from midwestern u s are explained by our model which yields similar magnetic enhancement factors for the two regions our model is also successful in predicting the mean annual rainfall threshold above which magnetic enhancement declines in a given type of climate

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

stratigraphy tectonics and palaeoclimatic implications of uranium series dated coral reefs from the sudanese coast of the red sea abstract new u th dates have been obtained from coral samples from the emerged reefs that border the sudan coast of the red sea the results show evidence of at least two reef formations the young formation situated at 2 6 m above m s l clearly corresponds to stage 5e of the last interglacial period according to the ages ranging from 125 8 to 142 9 ka this formation was coeval with that found all along the red sea coast from egypt in the north to ethiopia and djibuti in the south the old formation located at 8 5 m and 5 m above m s l has respectively been dated 300 ka and 253 53 35 ka these ages similar to those obtained on the egyptian coast suggest that the old reef was formed during the penultimate interglacial period field evidence no sign of folding or faulting and fairly constant height of the emerged reef deposits absence of reef formations dating stages 5c and 5a suggest that the sudan coastal area has been stable for at least 130 000 years two climatic phases much wetter than the present one were recorded one coeval with the formation of the young reef and the other immediately before the last regression period

oligocene range uplift and development of plateau morphology in the southern central andes 1 the puna altiplano plateau in south america is a high elevation low internal relief landform that is characterized by internal drainage and hyperaridity thermochronologic and sedimentologic observations from the sierra de calalaste region in the southwestern puna plateau argentina place new constraints on early plateau evolution by resolving the timing of uplift of mountain ranges that bound present day basins and the filling pattern of these basins during late eocene miocene time paleocurrent indicators sedimentary provenance analyses and apatite fission track thermochronology indicate that the original paleodrainage setting was disrupted by exhumation and uplift of the sierra de calalaste range between 24 and 29 ma this event was responsible for basin reorganization and the disruption of the regional fluvial system that has ultimately led to the formation of internal drainage conditions which in the salar de antofalla were established not later than late miocene upper eocene oligocene sedimentary rocks flanking the range contain features that suggest an arid environment existed prior to and during its uplift provenance data indicate a common similar source located to the west for both the southern puna and the altiplano of bolivia during the late eoceneoligocene with sporadic local sources this suggests the existence of an extensive longitudinally oriented foreland basin along the central andes during this time citation carrapa b d adelmann g e hilley e mortimer e r sobel and m r strecker 2005 oligocene range uplift and development of plateau morphology in the southern central andes tectonics 24 tc4011 doi 10 1029 2004tc001762

middle pleistocene climate change recorded in fossil mammal teeth from tarija bolivia and upper limit of the ensenadan land mammal age fossiliferous middle pleistocene sediments of the tarija basin of southern bolivia contain a classic ensenadan land mammal fauna new carbon isotopic data reported here for 50 specimens of the grazing mammals equus horse and cuvieronius mastodon documented from eight stratigraphic levels at tarija vary significantly in the 13c values of their teeth the pattern of variation appears to reflect the proportion of c3 and c4 grasses eaten during colder more c3 and warmer more c4 times within age limits set by associated magnetostratigraphy the cold periods can be correlated with particular even numbered stages in the marine oxygen isotope record and the warm periods can be correlated with odd numbered stages the oldest fossil teeth analyzed from the tarija section can thereby be assigned to stage 29 and the youngest to stages 17 or 15 that is the teeth range in age from about 1 1 myr to as young as 0 7 myr based on correlation of the upper part of the tarija beds to the isotopic stages the upper limit of the ensenadan land mammal age is between 0 7 and 0 6 myr which is younger than stated in most previous studies

dating quaternary raised coral terraces along the saudi arabian red sea coast abstract late pleistocene raised coral reef terraces form extensive outcrops up to 5 km wide along the saudi coast porites coral were dated using u th while clastic sediment from jeddah was dated using thermoluminescence the pooled mean age for the coral samples is 121 5 0 2 ka suggesting mis 5e even for the uplifted 16 20 m high terrace in the north at haql in jeddah the mis 5e back reef succession is overlain by fluvial sediment that gave a tl age of 66 13 ka the structure and faunal composition of the coral terraces suggests that they accumulated in broad shallow embayments following the last interglacial transgression the consistent elevation of these terraces suggests that the central and southern saudi coast has been tectonically stable for at least the past 125 000 years and the coral reef terraces at 3 5 5 5 m elevation are consistent with the mis 5e sea level high stand that peaked at 6 9 m above present sea level the saudi coastal coral terrace north of duba shows progressive uplift to 16 20 m near haql since 108 120 ka as a result of ongoing transform faulting in the gulf of aqaba

tropical atlantic sea surface temperature variability and its relation to el nino southern oscillation past analyses of tropical atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics across the intertropical convergence zone itcz by analyzing an improved 43 year 1950 1992 record of sst smith et al 1996 and other data derived from the comprehensive ocean atmosphere data set coads it is shown that the regions north and south of the itcz are statistically independent of each other at the seasonal to interannual timescales dominating the data confirming the conclusions of houghton and tourre 1992 some dipole behavior does develop weakly during the boreal spring season when there is a tendency for sst anomaly west of angola to be opposite of that in the tropical north atlantic it is further shown that tropical atlantic sst variability is corre lated with pacific e1 nifio southern oscillation enso variability in several regions the ma jor region affected is the north atlantic area of ne trades west of 40ow along 10on 20on and extending into the caribbean there about 50 80 of the anomalous sst variability is associ ated with the pacific enso with atlantic warmings occurring 4 5 months after the mature phases of pacific warm events an analysis of local surface flux fields derived from coads data shows that the enso related atlantic warmings occur as a result of reductions in the surface ne trade wind speeds which in turn reduce latent and sensible heat losses over the region in ques tion as well as cooling due to entrainment this enso connection is best developed during the boreal spring following the most frequent season of maximum enso anomalies in the pacific a region of secondary covariability with enso occurs along the northern edge of the mean itcz position and appears to be associated with northward migrations of the itcz when the north at lantic warmings occur although easterly winds are intensified in the western equatorial atlantic in response to pacific warm events they do not produce strong local changes in sst contrary to expectations from studies based on equatorial dynamics these teleconnected wind anomalies do not give rise to significant correlations of sst in the gulf of guinea with the pacific enso as the teleconnection sequence matures strong se trades at low southern latitudes follow the de velopment of the north atlantic sst anomaly and precede by several months the appearance of weak negative sst anomalies off angola and stronger positive anomalies extending eastward from southern brazil along 15o 30os

paleobiology and paleoecology of an arid semiarid miocene south american ichnofauna in anastomosed fluvial deposits abstract miocene deposits of the toro negro formation in la rioja province argentina host a rich vertebrate and invertebrate ichnofauna trace fossils are recorded from the lower part of the lower member of the toro negro formation at quebrada de la troya this succession consists of sandstone intraformational breccia mudstone and few conglomerate deposited in sandy anastomosing fluvial systems developed under a semi arid climate the invertebrate ichnofauna is composed of meniscate trace fossils taenidium barretti scoyenia gracilis dwelling structures palaeophycus tubularis and grazing trails helminthopsis hieroglyphica the vertebrate ichnofauna includes avian fuscinapeda sirin incumbent footprint slender anisodactyl footprint and mammalian footprints macrauchenichnus rector cf venatoripes riojanus small heteropod footprint kidney like footprints and oval impressions tetrapod footprints and rare palaeophycus and helminthopsis are preserved on the top of exposed sandbars crevasse splay deposits preserve abundant meniscate trace fossils and dwelling tubes e g taenidium scoyenia and palaeophycus ascribed to the scoyenia ichnofacies two suites of invertebrate trace fossils can be differentiated meniscate backfill structures t barretti and other burrows p tubularis without ornamentation developed in a soft substrate and striated traces s gracilis that crosscut the former developed in a firmer substrate the toro negro depositional system displays a high preservation potential for tracks due to seasonal flooding events which rapidly buried the footprints the favorable preservational conditions and good exposure of the track bearing surface enable us to interpret the track assemblage of the toro negro formation as a sample of the original tetrapod fauna that inhabited that region at central western argentina during the miocene the identification of possible trackmakers is utilized in this work as a useful tool that contributes to our understanding of the ancient environment the paleoichnocommunity is characterized by the number of trackmakers and the relative abundance of each type of footprints a new method to measure footprint abundance is proposed the relative bioturbed area rba this measurement provides numerical characterization of the degree of bioturbation of the whole tracking surface caused by each ichnotaxa the tetrapod fauna indicated by the footprint record was compared with the body fossil record of the toro negro formation these records overlap in some ways and in other ways they are complementary the presence of large and medium to small ground sloth proterotheriid litopterns caviid rodents and birds is pointed out by both body fossils and ichnofossils body fossils include remains attributed to dasypodids glyptodontids and abrocomid rodents ichnofossils indicate the presence of macraucheniid litopterns and three types of birds shorebirds perching birds and large cursorial birds

ice age vegetation and climate of subtropical chile abstract vegetation and climate of subtropical chile over the past 50 000 yr are reconstructed from pollen spores and other microfossils contained in a 10 7 m section of lacustrine deposits of laguna de tagua tagua 34 30 s 71 10 w controlled by 14 radiocarbon age determinations five pollen assemblages covering stratigraphic zones in the section represent plants on the upland and species inhabiting the lake basin surface pollen spectra from 58 stations approximately 30 40 s form an adjunct for interpreting past vegetation and climate from the fossil record temperature semi humid woodland of southern beech nothofagus dombeyi type and podocarp prumnopitys andina was apparently established about laguna de tagua tagua during the pleistocene contrasting existing semi arid broad sclerophyllous vegetation the woodland was extensive about the lake at the time of the last glacial maximum 25 000 14 000 yr b p and earlier 43 000 34 000 yr b p pollen from the families chenopodiaceae and amaranthaceae which characterize intervals of the pleistocene centered around 33 000 and 43 000 yr b p and the holocene suggests intervening episodes of relative aridity lake level fluctuations appear to follow the late quaternary climatic pattern implied by pollen assemblage data water in the lake was relatively high during development of southern beech podocarp woodland whereas during intervals of chenopod amaranth dominance lake levels were comparatively low excess precipitation over evaporation which during the pleistocene favored development of woodland of beech and podocarp is attributed to greater storm frequency conditions were apparently governed by strengthening of atmospheric circulation in the belt of southern westerly winds in the late glacial the temperate and more humid ice age climate with limited seasonality underwent transition to a subtropical semi arid mediterranean type that identifies the postglacial fluctuations in intensity of the southern westerlies marked by latitudinal movement of the polar front can account for past vegetation changes at laguna de tagua tagua

comparison of electrojet indices from the northern and southern hemispheres a unique data set of digital and digitized analog magnetic recordings from 22 stations in the antarctic was used to construct the southern hemisphere equivalent of the northern hemisphere auroral electrojet index ae for two separate intervals of magnetic disturbance totaling seven days in june 1982 a second index constructed using only 9 stations between approximately 60 degrees 70 degrees geomagnetic south latitude showed only small differences from that using all 22 stations for the universal time interval 00 11 ut inclusive when a reasonably good coverage of ground stations exists in the austral auroral zone it is found that there is a good correlation between the northern and southern hemisphere indices this is the case even though the southern ionosphere is largely in total darkness during the interval studied no effect of the north south direction of the interplanetary magnetic field is found on the correlation

auroral electrojet indices in the northern and southern hemispheres a statistical comparison the auroral electrojet ae index is traditionally derived from about 12 ground magnetometer observatories located around the average northern auroral oval location the ae index calculation has only been performed with northern hemisphere data because similar coverage in the southern hemisphere does not exist in this study eight southern auroral ground magnetometers and their near conjugate northern hemisphere counterparts are used to calculate conjugate ae indices for 274 days covering all four seasons from 2005 to 2010 the correlation coefficient between the northern and southern ae indices for many of the intervals is 0 65 indicating strong asymmetries between the two hemispheres we compare our conjugate ae indices with the standard ae index and find a number of asymmetries because of station coverage gaps in the southern and northern arrays the mean difference between the southern and northern ae indices is largest during northern summer season and the smallest mean difference occurs in the spring the mean differences between the southern and conjugate northern ae indices are about 31 nt with the largest differences occurring in the midnight magnetic local time mlt sector we suggest that these differences may be a function of seasonal mlt and ionospheric effects we also find a difference in the southern and northern ae related to ut and believe that this pertains to the distribution of the magnetometers the fact that a difference between the southern and northern ae indices exists indicates the importance of examining geomagnetic activity in both hemispheres when considering magnetospheric phenomena

magnetostratigraphy and magnetic parameters of a sedimentary sequence in punta san andres buenos aires argentina abstract this contribution focuses on polarity changes and magnetic parameters obtained in late cenozoic sediments exposed on the cliffs in san andres chapadmalal buenos aires province the sedimentary section studied comprises the vorohue san andres miramar and arroyo seco formations the upper layers of vorohue and the base of san andres showed records of normal polarity and were attributed to the gauss normal polarity chron 2 6 ma the main portion of san andres formation appears to have been deposited during lower and middle matuyama polarity chron including the normal polarity subchron olduvai 1 78 2 02 ma miramar formation contains polarity changes that should be assigned to middle matuyama jaramillo 1 05 ma and to jaramillo upper matuyama 0 99 ma the upper portion of the miramar formation and the lower one of the arroyo seco formation show reverse polarity levels and were assigned to the upper matuyama 0 78 ma finally the upper part of arroyo seco formation seems to have been deposited during the brunhes polarity chron

the cretaceous tertiary boundary event in ecuador reduced biotic effects due to eastern boundary current setting a multidisciplinary study of a new cretaceous tertiary k t boundary section near guayaquil ecuador reveals an unusually cool water low diversity planktic foraminiferal fauna and a high diversity radiolarian fauna similar to those found in southern high latitude k t sequences despite the fact that this section was deposited near the cretaceous equator the k t boundary is located by planktic foraminifera within a narrow interval bounded by last appearances of tropical cretaceous species and first appearances of tertiary species including parvularugoglobigerina eugubina as in southern high latitudes there is no major mass extinction of either planktic foraminifera or radiolarians at this level a major radiolarian faunal discontinuity occurs some 6 m higher in the section within foraminiferal zone plc some 300 500 kyr after the k t event 13c values from bulk carbonates show both high and low latitude characteristics similarly to low latitudes there is a 3 negative 13c excursion at the k t boundary which is generally interpreted as a major decrease in primary productivity but unlike the low latitudes recovery occurs within a few thousand years as compared with 300 500 kyr and suggests rapid nutrient influx from the antarctic region via a current similar to the humboldt current today similarly to high latitude k t sequences a negative 13c shift occurs in the early danian zone plb about 300 kyr after the k t boundary sedimentologic and mineralogic data indicate a late maastrichtian with relatively low biogenic quartz and high carbonate followed by increasing biogenic quartz 50 and decreasing carbonate 5 during the early danian this suggests intensified atmospheric and oceanic circulation and upwelling off ecuador during the early danian the k t transition is marked by increased volcanic activity continental erosion and terrigenous influx but this also occurs in the early danian zone at the p1ap1b zonal transition and is thus not unique to the k t boundary we suggest that the catastrophic biotic effects normally observed at the k t boundary in low latitudes are greatly reduced or absent in the eastern equatorial pacific because this region was dominated then as now by upwelling and current transport of nutrient rich waters from the antarctic ocean as a result the biotic patterns are characteristic of southern high latitudes whereas the 13c pattern combines ameliorated low latitude effects with predominantly high latitude trends

antarctic peninsular cryosphere early oligocene c 30 ma initiation and a revised glacial chronology strontium isotope stratigraphy provides a chronology for cenozoic cryogenic strata in the northern antarctic peninsula and allows an assessment of diachronism in onset of glacial conditions between east and west antarctica the earliest observed event on the antarctic peninsula was late early oligocene 29 8 0 6 ma at least c 4 ma later than in east antarctica with a second in the early early miocene 22 6 0 4 ma both glacials and intervening interglacial were continent wide phenomena two late neogene glaciations are distinguished early late miocene 9 9 0 97 ma hobbs glacier jones mountains and latest miocene alexander island a further late neogene glacial deposit weddell formation can be constrained only as c 10 ma

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

tephrochronology and tephrostratigraphy of two pleistocene continental fossiliferous successions from central italy the stratigraphical context of two middle pleistocene fossiliferous palaeosols from central italy abruzzo and tuscany have been studied small mammals and molluscs occur in both palaeosols which are covered by tephra layers that were analysed using an interdisciplinary approach application of fission track dating to apatites separated from the case picconetto tephra pescara abruzzo yielded an age of 0 48 0 04 ma indistinguishable from those previously determined for the campani quarry lower valdarno tuscany 0 46 0 05 ma and 0 48 0 05 ma geochemical and petrographic investigations indicate that these tephra originated from different volcanoes the alban hills volcanic complex and the vico volcano latium respectively small mammal and mollusc assemblages indicate different palaeoclimatic and palaeoenvironmental conditions for the case picconetto and campani quarry palaeosols warm and humid conditions can be inferred for the campani quarry site whereas open and cold conditions can be inferred for case picconetto on the basis of faunal data fission track dates and attribution of tephra to specific volcanic eruptions we suggest a correlation of these faunas with marine oxygen isotope stage 14 case picconetto and with marine oxygen isotope stage 11 campani quarry respectively copyright 2003 john wiley sons ltd

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

magnetic parameters reflecting pedogenesis in pleistocene loess deposits of argentina abstract quaternary loess sequences of argentina with interbedded loess and buried soils palaeosols provide terrestrial records of past climates and environmental conditions study of rock magnetic parameters measured over a large area of the pampean loess seems to indicate that the existing magnetoclimatological models cannot adequately account for the complexities of the pampean loess the chinese loess has been considered as typical where magnetic properties are largely controlled by pedogenesis on the other hand the siberian loess is an alternative magnetoclimatological model in which palaeosols appear as magnetic lows and the intercalated loess as magnetic highs argentine loess is apparently closer to the siberian model however considering the data obtained in argentina the situation seems to be more complicated the higest magnetic values sirm and susceptibiliy values in silty and sandy loess indicate a more efficient entrainment of dense iron oxides particles during stormy dry glacial intervals the parent material shows the highest susceptibility values 100 10 8 m3 kg while the waterlogged horizons show the lowest ones below 20 10 8 m3 kg pedogenesis resulting in the development of bc and b soil horizons of palaeosols in the parent loess produced decreases in susceptibility values and increases in the f factor this phenomenon occurs at some degree of humidity in which the process of gleying caused the total depletion of both susceptibility and frequency factor the magnentic data allows consideration of the relevance of major cycles arid humid separated by discontinuities as the main factor favoring one particular behavior of the magnetic parameters the b horizons of palaeosols developed during an arid cycle will not show a notable difference in the magnetic records from the parent material in contrast during humid climate condition the pristine loess can be progresively obliterated by pedogensis through to the extreme situation of gleying

comparison of auroral electrojet indices in the northern and southern hemispheres 1 the auroral electrojet ae index is traditionally calculated from a set of about 10 to 13 ground magnetometer stations located around the typical northern auroral oval location similar coverage in the southern hemisphere does not exist so the ae calculation has only been performed using northern hemisphere data in the present paper we use seven southern auroral region ground magnetometers as well as their conjugate northern hemisphere data to calculate conjugate ae indices during the northern hemisphere winter using the standard method the correlation coefficient between the northern and southern ae indices for many of the intervals is above 0 7 but in one interval it is close to 0 we compare our conjugate ae indices with the standard ae index and find a number of asymmetries because of station coverage gaps both in our conjugate indices and in interplanetary magnetic field imf conditions when the southern ae index is compared with the standard ae index we find that for most intervals the correlation is less than 0 7 and in one interval the correlation is about 0 the correlation between the conjugate northern ae index and the standard ae index is somewhat better suggesting true interhemispheric asymmetries the mean difference between the southern and northern ae indices is largest during southward imf and for large values of imf by 5 nt this is most likely due to the increased activity levels during southward imf and the greater twisting of the magnetic field lines during strong imf by imf bx seems to have no effect on the interhemispheric differences of the ae index the mean differences between the southern and conjugate northern h component are of the order of 35 nt with the largest differences occurring in the midnight magnetic local time mlt sector we suggest that these differences may be related to seasonal effects ionospheric effects and mlt effects

geological pedological and paleomagnetic study of the late cenozoic sedimentary sequence in southwestern uruguay south america abstract the results of interdisciplinary studies carried out on the coastal cliffs of southern uruguay are presented here the raigon formation late pliocene middle pleistocene has a cratonic provenance and comprises two sections limited by a regional discontinuity each section is composed of an association of lithofacies ss sp sr rare st and fm the last one represents floodplain deposits and the others are interpreted as channel filled deposits in the upper part a well structured paleosol with clay coatings and abundant rhizoliths is observed in a sharp contact with silty deposits of the libertad fm early middle pleistocene these sediments have a mixed provenance magmatic arc and cratonic sheet like geometry thick stratification and vertical walls and are composed of lithofacies fr these characteristics suggest a loessic origin a paleosol with illuvial b horizons and silty loam texture is recognized at the bottom of this unit the recent soil argiudoll is developed at the top of the libertad fm the characteristics of the sequence indicate climatic fluctuations and several cycles of erosion sedimentation and pedogenesis according to magnetostratigraphy the chronology is roughly compatible with that one based on the fossil contents and is in agreement with magnetic ages determined in argentine fossiliferous sequences