modern reindeer and mice revised phosphate water isotope equations abstract the oxygen isotope composition of bone and tooth phosphate of 34 mice specimens pitymus sp microtus arvalis and arvicola terrestris coming from seven different locations in italy germany and switzerland was measured by means of well established techniques these measurements were carried out with the purpose of establishing quantitative relationships between the 18op from different mice genera and the mean 18ow values and to compare these data to previous measurements carried out on various specimens belonging to the genus apodemus the three genera studied showed a similar behaviour when compared to the mean 18ow values the slope of the equation calculated for these three genera is significantly different from the slope obtained from apodemus specimens reconsidering the 18ow values suggested in the case of apodemus due to the small number of data available at that time it seems that these values are too negative by 0 5 to about 1 5 if so the apodemus equation becomes almost identical to the equation calculated for the new mice values and consequently one could conclude that several micromammal genera and species might behave in the same way and obey the same relationship with the mean 18ow values a set of 25 samples of modern reindeer skeletal material from spitzbergen russia and siberia was also studied with the aim of improving the reindeer isotope equation obtained from a previous study in fact the slope of that equation was somehow uncertain due to a rather large range of isotope values obtained from each group of reindeers coming from the same location the new results confirm such anomalous behaviour already shown by other mammals and probably related to dietary behaviours water fluxes with the environment and isotopic composition of ingested food and water rather than to imperfect equilibrium conditions with environmental water however the equation calculated from both the old and new statistically more significant isotope results is not far from the previous one

late quaternary paleosols stratigraphy and landscape evolution in the northern pampa argentina the field properties micromorphology grain size geochemistry and optically stimulated luminescence osl ages of two late quaternary sections have been used to reconstruct the sequence of pedosedimentary processes and to provide insights into landscape evolution in part of the northern pampa of argentina paleosols developed in paludal sediments adjacent to the parana river at baradero and in loess at lozada can both be correlated and linked to other sites thus enabling for the first time the tentative recognition and tracing of a diachronous soil stratigraphic unit that probably spans the equivalent of at least part of marine oxygen isotope stage ois 5 the paleosol at lozada was truncated and buried beneath fluvial sediments during the time span of ois 4 and 3 eolian gradually replaced paludal inputs at baradero over this period and there were also two clearly defined breaks in sedimentation and development of paleosols the period corresponding to ois 2 was marked by significant loess accumulation at both sites with accretion continuing into the mid holocene only at lozada the more developed nature of the surface soil at baradero probably reflects a combination of a moister climate and a longer soil forming interval

postglacial rebound observed by lageos and the effective viscosity of the lower mantle sixty four observations of the orbital node made by the lageos satellite over a five year time interval reveal an acceleration of 8 1 or 1 8 x 10 to the 8 power arcseconds day 2 due to a source which is not presently modeled in the geodyn orbit determination computer program this acceleration cannot be explained by the ocean tide with 18 6 year period assuming it to be an equilibrium tide instead it seems to be due to postglacial rebound which changes the j 2 coefficient in the spherical harmonic expansion of the earth s gravitational field at the rate of 8 2 or 18 x 10 to the 19th power s this in turn accelerates the node this rate does not agree with the 32 x 10 to the 19th power s predicted by wu and peltier s 1982 l2 model which has upper and lower mantle effective viscosities of 10 to the 21st and 22nd powers pa s respectively it does agree well with their l1 model which gives about 10 x 10 to the 19th power s since the effective viscosity is 10 to the 21st power pa s throughout the entire mantle in the l1 model the results support the contentions that the efective viscosity is near 10 to the 21st power pa s everyhere in the mantle and this relatively low value for the effective viscosity may have permitted several degrees of polar wander due to glaciation during the quaternary ice age previously announced in star as n84 13705

whole mantle convection and the thermal evolution of the earth abstract three recent results in the fields of geophysics and geochemistry have combined to rejuvenate the study of the earth s thermal history the first and perhaps most important of these is the new understanding of the planetary accretion process which has been realized as a consequence of work by v s safronov and others this work shows that the terrestrial planets accrete on a time scale of 10 8 y by the accumulation of planetesimal sized objects and that they probably form sufficiently hot to induce immediate differentiation of iron from silicates the second important recent contribution has been from geochemistry and concerns the net complement of radioactivity of the earth as a whole although the net complement of radioactive elements has usually been inferred on the basis of the assumption of equilibrium with the surface heat flow a priori analysis based upon elemental condensation temperatures in the primitive solar nebula yields a model in which the net radioactivity is at least a factor of two less than would be required to explain the mean surface heat flow there are good reasons to believe that this model is valid for the earth the third result has been the successful development of a technique for incorporating the influence of convective heat transfer in a thermal evolution model which does not require explicit solution of the dynamical equations this paper begins with an analysis of the mantle convective circulation which is required to understand plate tectonics and continental drift an analysis which presents the arguments for and against the crucial notion that the main circulation has a depth scale equal to the mantle thickness using the idea of parameterized convection the radial heat transfer by the mantle convective circulation is included in a thermal history model of the earth this model turns out to contain within it an interesting explanation of the fact that no crustal rocks with ages in excess of 3 8 ga have ever been found an observation that has heretofore proven enigmatic

mantle phase transitions and the free air gravity anomalies over fennoscandia and laurentia the interpretation of free air gravity anomalies over present day centers of postglacial rebound has always been controversial we show that the observed gravity anomalies over both fennoscandia and laurentia and the associated relative sea level variations since deglaciation are explicable with a simple newtonian viscoelastic model of the earth if the earth model includes the density jumps in the mantle associated with the phase transitions at 420 and 670 km depth the result has important implications for mantle convection since the observed free air anomalies are very sensitive to the viscosity of the mantle beneath the deepest phase transition

the thickness of the continental lithosphere two independent sets of geophysical data are shown to be particularly sensitive to the thickness of the continental lithosphere both are connected with the response of the planet to the last deglaciation event of the current ice age the first set of data consists of relative sea level histories in the age range 0 10 kyr b p from sites along the eastern seaboard of north america such data appear to require a continental lithospheric thickness in excess of 200 km the second set of data consists of the ils record of the motion of the rotation pole over the time range 0 75 years b p which reveals a secular drift at the rate of 0 95 0 15 deg 106 years toward hudson bay when the viscosity profile of the mantle is fixed to that required to explain free air gravity and relative sea level data from within the ice margin and the observed nontidal acceleration of planetary rotation then these observations also suggest a continental lithospheric thickness of the same order

new radiocarbon chronology for the guerrero member of the lujan formation buenos aires argentina palaeoclimatic significance abstract new radiocarbon dating performed on materials from the guerrero member of the lujan formation considerably enlarges the time period previously assigned to this unit new data suggest that deposition began about the last glacial maximum ca 21 ka bp and continued until at least ca 10 ka bp during this period several climatic and biological events occurred among the latter the last processes leading to the extinction of pleistocene megafauna

nature of environmental changes in south america at the last glacial maximum abstract a review is given of geomorphological evidence suggesting the nature of environmental changes in south america during the last glaciation maximum lgm the data are from glacial geocryogenic alluvial colluvial and aeolian systems and span 68 of latitude making the most complete land transect of major climatic systems in the southern hemisphere glaciers in the northern and central andes may have reached their maximal extent by 27 000 yr b p reduced precipitation at the lgm caused by lower temperatures and lower atmospheric humidity probably led to slight glacier recession in the tropical andes icefields in the southern andes were most expansive when global temperature and sea level were lowest because pacific westerlies supplied abundant moisture and western outlets advancing across the continental shelf were constrained by sea level low gradient river networks became incised by at least 100 m into narrow channels as global sea level fell by 120 m during the lgm draw down of water tables possibly impacted the forest cover enhancing the drying influence of reduced sea surface temperature and atmospheric humidity as forest and grass covers diminished in extent effecting greater atmospheric cooling because of reduced evapotranspiration and convective condensation colluvial and aeolian processes became more active and widespread almost 25 of the continent is covered with palaeo aeolian features presumed to have been active when airflow was stronger due to steeper atmospheric pressure and temperature gradients at the lgm substantial changes in precipitation totals and incidence and in ground water availability probably eliminated modern type tropical rainforest except in areas currently receiving ca 5000 mm of annual rainfall equivalent changes in vegetation cover spread across much of the guyana and brazilian highlands and throughout the southern temperate regions

first record of dasyproctidae rodentia in the pleistocene of argentina paleoclimatic implication abstract plesiaguti totoi gen et sp nov the first member of the dasyproctidae recorded in the pleistocene of argentina is described it comes from the coastal cliffs of necochea on the southern coast of buenos aires province taking into account its tooth morphology pentalophodont and almost brachyodont it represents one of the most primitive dasyproctids the morphology of the occlusal surface suggests that p totoi is more closely related to the living representatives of the family than to extinct species of the oligocene miocene of argentina this phylogenetic hypothesis plus the fact that dasyproctids vanished from these latitudes by at least the early pliocene argues that p totoi is an immigrant to buenos aires province its presence reinforces the idea of the existence of one or more warm pulses during the pleistocene that permitted the southward migration of brazilian fauna

predominance of arid climates indicated by mammals in the pampas of argentina during the late pleistocene and holocene abstract south american climates during the pleistocene and holocene are poorly known especially in the lowlands east of the andean ranges however there is relatively good biostratigraphic and climatic information for the late lujanian and platan local ages latest pleistocene to holocene of the pampas region most of the climatic data from the pampean area are based on land mammal assemblages pollen records and geology contrasting with north american faunal associations there are many non analogue assemblages in the holocene of the pampean area the climate was arid and relatively cold during most of the early lujanian however several relatively wet and warm events are suggested by the presence of subtropical mammals and evidence of tchernozoid paleosoils the late lujanian is well represented by the fossiliferous sediments of the la chumbiada and guerrero members of the lujan formation ca 30 000 and 21 000 13 000 yr b p respectively cropping out in stream valleys in the buenos aires province mammal assemblages suggest a change from a temperate to a cool climate from the la chumbiada member to the guerrero member the latter representing the last glacial maximum the lower portion of the eolian la postrera formation has been dated at 11 000 8000 yr b p this section was deposited on basin divides under dry and cool conditions the mammal association was similar to that of the gueffero member and includes the last megaherbivore mammals in south america e g megatherium americanum mammals occurring in platan beds usually indicate dry but not cool conditions warm and probably wetter conditions were detected in continental beds deposited synchronously with a mid holocene marginal marine ingression ca 7000 6000 yr b p a holocene non analogue assemblage of subtropical mammals and dry cold indicators occurred at latitude 38 30 s at ca 2000 yr b p subtropical mammals that require wetter conditions have been migrating southwards in the pampean region since 1500 1000 yr b p eolian sediments in the la postrera formation dated ca 440 yr b p yielded an arid to semiarid mammal assemblage which could relate to the little ice age period remarkably most of the present day manual biocenoses and climatic conditions in the pampas seem to have been recently established paleogeographic maps for the last glacial maximum and the mid holocene are also provided

the lageos constraint on deep mantle viscosity results from a new normal mode method for the inversion of viscoelastic relaxation spectra the recently observed acceleration in the node of the orbit of the lageos satellite has previously been shown to provide a good constraint on the viscosity of the earth s mantle beneath the 670 km seismic discontinuity this paper explores the extent to which the previously inferred lower mantle viscosity may be traded off against variations of other properties of the radial viscoelastic structure we show explicitly that the nontidal acceleration of rotation inferred from the lageos data and predicted as a consequence of the earth s response to the last deglaciation event of the current ice age is relatively insensitive both to variations of lithospheric thickness and to the presence or absence of plausible chemical heterogeneity across the 670 km interface it therefore provides a high quality constraint on the viscosity of the deep mantle with the upper mantle viscosity fixed at the nominal value of 1021 pa s the bounds on lower mantle viscosity lm are such that 2 1021 pa s lm 6 1021 pa s even when allowance is made for the presence of a viscous boundary layer at the base of the upper mantle such as would exist if the mantle convective circulation were layered

stable isotope analyses on the last 30 ka molluscan fauna from pampa grassland bonaerense region argentina abstract fossil and living shells of terrestrial and freshwater molluscs collected from various sites in the bonaerense and patagonia regions of argentina were analysed for their stable isotope content as a possible tool for palaeoclimatic and palaeoenvironmental reconstruction the oxygen isotope composition of calcareous shell from freshwater samples was interpreted in terms of a change in moisture condition and the related evaporative effect on surficial bodies of water linked to a climatic shift the carbon isotope composition mainly records changes in biological productivity interpretation of the isotopic composition of the land snail carbonate is more complex and worldwide data from living shells have been used to decipher to some extent the palaeoenvironmental record contained in their isotopic composition overall the interpretations deduced from isotopic data of land snails as well as from nearby freshwater molluscs are quite comparable the isotopic data suggest that arid conditions occurred between 35 and 15 ka b p and warmer conditions around 35 25 ka b p a shift toward a new phase comparable with the present meteorological and climatic situation probably began around 9 ka b p and it was linked to some short but severe climatic fluctuations thus suggesting that high climatic stress characterised this period moreover a trend toward slightly arid conditions is suggested by isotopic data possibly sometimes after 5 ka the differences arising from the geographical location and the short duration of each single series studied still hamper an extensive interpretation of the isotopic data

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

landscape evolution and climate changes in the late pleistocene holocene southern pampa argentina evidence from palynology mammals and sedimentology abstract the landscape evolution of the southwestern part of buenos aires province southern pampa is a good example for the understanding of continental deposits in arid semiarid regions in buenos aires province the last 9 3 ma record is described as sequences composed of alternating loess and palaeosol units indicating climatic fluctuations between arid and cold loess deposition and warm and humid intervals palaeosol development the present drainage system flows across the early pliocene middle pleistocene stratigraphic units the stratigraphy and evolutionary history are analyzed using sequence stratigraphic criteria the interpretation of the climatic evolution of the area is based on pollen and mammal mainly rodents records the late pleistocene sediments reflect arid to semiarid conditions the late pleistocene holocene transition is characterized by development of palaeosols the early holocene pollen records reflect the development of a vegetation community characteristic of coastal dunes sea level was still lower than today temperature and humidity reached its maximum during the mid holocene when the high diversity and abundance of marine dinocysts and acritarchs indicate a transgression this event is associated with the gramineous steppe in the continent reflecting more temperate or local humid conditions and with brazilian mammal fauna this relative rise of sea level lead to flooding the riverbeds producing deposition of gray muddy facies approximately at 3000 years bp the marine influence ended in the area after 2610 years bp psammophytic herbaceous steppe development as well as mammals suggests arid to semiarid conditions an interval of higher humidity is inferred at approximately 2000 years bp based on the development of gramineous steppe communities a relative rise of temperature may be inferred by the southward expansion of the brazilian mammal fauna

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

the campo laborde site new evidence for the holocene survival of pleistocene megafauna in the argentine pampas abstract investigations have been carried out at the campo laborde archaeological site in the argentine pampas some issues related to pleistocene extinctions and the survivals of some species of megafauna into the early holocene are discussed at the site of campo laborde abundant bones of giant ground sloth megatherium americanum some from other pleistocene fauna doedicurus sp and neosclerocaliyptus sp and very few from extant fauna e g vizcacha lagostomus maximus patagonian hare dolichotis patagonum and peccary tayassu sp were found in association with two quartzite tools 128 microlithic debris from different raw material and two informal bone tools each of the species is represented by only one individual and only one bone of m americanum and one of d patagonum show cut marks different lines of evidence suggest that campo laborde is a giant ground sloth procurement site on the border of an ancient swamp where the prey was also initially butchered one of the recovered tools is the stem of a broken bifacial projectile point which would have been used as a hunting weapon based on the stratigraphic position this site could date to the early holocene six ams 14 c ages from m americanum bone collagen support this age although there is a wide chronological span between ca 9700 and 6700 bp the evidence obtained from campo laborde as well as from the la moderna site indicates that some pleistocene species such as giant ground sloth and some glyptodon d clavicaudatus and neosclerocaliyptus sp survived in the pampas until the early holocene therefore these megafauna coexisted with early hunter gatherers for several millennia and were exploited

late quaternary vegetational and climatic changes in the pampa grassland of argentina abstract the vegetation and climate of the pampa grassland argentina during the late quaternary are reconstructed from pollen recovered from dated stratigraphic sections from arroyo walls and from archaelogical excavations prior to 10 500 yr b p herbaceous psammophytic steppe existed in the central part of the pampa grassland while xerophytic woodland associated with psammophytic and halophytic steppe occurred in the southwestern part of the pampa these types of vegetation and the continental conditions that prevailed in the area of the present day coast 38 s indicate subhumid dry climate and annual precipitation 100 mm lower than present a subsequent change toward a vegetation characteristic of ponds swamps and foodplains or toward environments with locally more effective moisture occurred ca 10 500 yr b p suggesting annual precipitation close to modern levels or a higher availability of water in the central part of the pampa grassland this type of vegetation existed until 8000 yr b p when it was replaced by grassland communities that lasted until 7000 yr b p in the southwestern part of the pampa grassland this vegetation developed before 7000 yr b p and persisted until ca 5000 yr b p sea level higher than the present ca 6200 yr b p is consistent with sharp modification of the vegetation and development of local halophytic communities dominant at 38 s a return to subhumid dry conditions occurred after 5000 yr b p the late holocene vegetation is characterized by pollen assemblages similar to the psammophytic and halophytic communities of the southern pampa grassland associated with communities with more edaphic conditions at the same time at 38 s a sea level regression is suggested by the dominance of fresh water pollen assemblages and micropaleontological remains the trend toward humid temperate conditions ca 1000 yr b p suggested by vertebrate remains characteristic of temperate and humid conditions as well as a very short but dry episode during the 18th century suggested by the geology are not clearly evidenced in the pollen sequences vegetational and climatic changes are explained by the latitudinal shifts and changes in intensity of the southern atmospheric circulation and changes in sea level

quaternary mollusc assemblages from the lower basin of salado river buenos aires province their use as paleoenvironmental indicators abstract quaternary mollusc assemblages preserved in the salado basin are described and analyzed in order to reconstruct past environments and evaluate spatial and temporal variations five localities were analyzed in which 48 998 individuals of eight continental gastropod species were recovered heleobia parchappii antillorbis nordestensis biomphalaria peregrina drepanotrema heloicum uncancylus concentricus pomacea canaliculata succinea meridionalis and miradiscops brasiliensis as well other species typical of marine estuarine environments have been found among which heleobia australis is highlighted based on changes in the composition and abundance of the assemblages different environments could be recognized quaternary mollusc assemblages have low taxonomic heterogeneity and spatial and temporal variations assemblages recovered from late pleistocene early holocene sediments are characterized by low species richness and abundance with a single species in numerous examples in the middle holocene the shells became more constant and more diverse late holocene assemblages have high species richness freshwater terrestrial and amphibious habits and show some differences among different localities several episodes of flooding have been identified through the record of fossiliferous horizons with high density of shells mainly heleobia parchappii these events that affected the area of influence of the salado river occurred at least during the late holocene

continental lithospheric thickness and deglaciation induced true polar wander the secular drift of the earth s rotation pole which is evident in ils data is a memory of the planet of the last deglaciation event of the current ice age in order to fit the observed rate and direction of drift with an earth model whose viscosity profile is capable of reconciling the free air gravity and relative sea level data of postglacial rebound requires a lithospheric thickness which is substantially in excess of that which might be considered an appropriate average thickness for the entire planet this required thickness which is greater than 250 km is nevertheless in accord with independent estimates of the thickness of the continental lithosphere

higher harmonic effects of the earth s gravitational field from post glacial rebound as observed by lageos recently long term orbital data from the artificial satellite lageos have been analyzed to determine the rate of post glacial rebound rubincam has compared the observed values against values derived from two realistic earth models wu peltier l1 l2 working with the second harmonic mode of excitation the data support model l1 with a uniform mantle viscosity of 1022 poise over l2 with a lower mantle viscosity of 1023 poise we find the higher degree harmonic contributions are significant and considerably strengthen the support for model l1 we perform such calculations using the same parameter values for glaciation as rubincam and also values due to peltier and coworkers we also show that a similar analysis of data from the satellite starlette is less likely to be as definitive as analysis of lageos data since there are significant contributions from very high harmonics indicating that the behavior of the earth must be known in much greater detail

pleistocene deglaciation and the global gravity field the present day global geoid and free air gravity anomaly signals induced by pleistocene deglaciation have been computed along with their secular variations a revised green function has been derived for the free air gravity anomaly and we find that an earth model with a lower mantle viscosity moderately larger than the upper mantle value satisfies observations of the anomaly over both hudson s bay and fennoscandia using this preferred earth model the predicted global geoid anomaly map is characterized by peak negative values of 36 7 9 4 and 21 9 m over hudson s bay northern europe and antarctica respectively and by a smooth small amplitude 2 5 m upwarping over the major ocean basins although the field has an order of magnitude less power than the observed there is a significant spatial correlation between the two inferences of mantle viscosity based upon comparisons between the observed geoid and the computed deglaciation induced anomaly are hampered by the large contribution from the convective circulation in the earth s mantle we have subtracted a predicted convection signal based on seismic tomography from the observed field however the residual geoid thus obtained is still too inaccurate to allow a useful comparison in contrast a filtered version of the observed geoid over canada which retains harmonic coefficients in the degree range 10 22 provides a relatively uncontaminated by convection datum that is indeed satisfied by the preferred earth model secular variations in the earth s gravitational field are naturally dominated by the signal from pleistocene deglaciation in this respect we compute zonal harmonics in the expansion of the present day rate of change of the geopotential and in particular we predict amplitudes comparable to that for the previously analysed j2 for j4 j5 j7 and j9 all of which reflect in large part the response due to the melting of the massive laurentide ice sheet this analysis suggests that measurement of these higher order secular variations of the zonal harmonics based upon analysis of long time series of lageos ranging data could provide very useful additional constraints on the radial variation of mantle viscosity these constraints along with all others described in this paper are shown to be relatively insensitive to assumptions concerning the deflection during the glacial isostatic adjustment process of the 400 and 670 km density discontinuities within the mantle

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

paleobiology of sabretooth cat smilodon populator in the pampean region buenos aires province argentina around the last glacial maximum insights from carbon and nitrogen stable isotopes in bone collagen abstract the sabretooth cat smilodon populator was the largest felid in south america it appears in the fossil record in the early pleistocene as an immigrant from north america and becomes extinct around the pleistocene holocene boundary the carbon and nitrogen stable isotopic values of collagen were measured for skeletal remains from smilodon specimens ranging in age from 25 to 10 kyr bp for the first time in the pampas region of argentina by comparison with similar values obtained on co eval predators such as protocyon large canid and panthera onca jaguar and a range of potential prey such as giant ground sloths glyptodontids macrauchenia toxodon equids cervids and rodents it could be established that smilodon consumed essentially large prey from open landscape such as macrauchenia and giant ground sloths during the last 15 000 years of the late pleistocene in the pampa region it was possibly competing with the large canid protocyon but the jaguar was apparently feeding on smaller size prey a more humid climate at the beginning of the holocene might have been unfavorable to this large predator and could have contributed to its extinction these results also provide an important insight to understand the ecological processes involved in the great american biotic interchange

environmental history since 11 000 14c yr b p of the northeastern pampas argentina from alluvial sequences of the lujan river abstract sedimentological malacological and pollen analyses from 14c dated alluvial sections from the lujan river provide a detailed record of environmental changes during the holocene in the northeastern pampas of argentina from 11 200 to 9000 14c yr b p both sedimentary and biological components suggest that the depositional environment was eutrophic alkaline and freshwater to brackish shallow water bodies without significant water circulation during this time bioclastic sedimentation was dominant and the shallow water bodies reached maximum development as the climate became more humid suggesting an increase in precipitation short term fluctuations in climate during the last stage of this interval may have been sufficient to initiate changes in the water bodies as reduction of the volume alternated with periods of flooding the beginning of the evolution of shallow swamps in the wide floodplain or huge wetlands was contemporaneous with a sea level lower than the present one from 9000 and 7000 14c yr b p mesotrophic alkaline brackish probably anoxic swamps existed between 7000 and 3000 14c yr b p anoxic calcareous swamps were formed with subaerial exposure and development of the puesto berrondo soil 3500 2900 14c yr b p a trend to a reduction of water bodies is recorded from 9000 to ca 3000 14c yr b p with a significant reduction after ca 7000 14c yr b p a shift to subhumid dry climate after 7000 14c yr b p appears to be the main cause during this time an additional external forcing toward higher groundwater levels was caused by holocene marine transgression causing changes in the water bodies levels the climate became drier during the late holocene ca 3000 yr b p when clastic sedimentation increased under subhumid dry conditions flood events increased in frequency during this time from ca a d 1790 to present the pollen record reflects widespread disturbance of the vegetation during the european settlement

the argentine neotropical loess an overview abstract the neotropical loess primary and reworked covers a large part of argentina north of 30 s it is different in some ways from the loess of the pampa which is generally considered the typical argentine loess the mineralogical chemical and physical composition of the neotropical loess has been analyzed and is here compared with both pampean and northern hemisphere european and north american loess the distribution of neotropical loess is established following the former teruggi scheme and the bio stratigraphical data indicating a late pleistocene age is described the great climatic variability of this period is documented through description of a loess paleosol sequence that mantles the pre andean valleys the influence of loess influx upon the genesis of the paleosols and the stadial interstadial environmental changes are considered in relation to the generalized truncation of paleoedaphic sequences the migration of the polar front and the weakness of the pacific high pressure system are also analyzed in relation to south american environmental evolution during the upper quaternary in that sense the genesis of pre andean fluvial valleys and mega fans extending outward into the andean piedmont plains having sources on the eastern dry side of the andes would have been influenced by such a paleoclimatic situation

new late pleistocene megafaunal assemblage with well supported chronology from the pampas of southern south america abstract late pleistocene outcrops exposed in buenos aires province argentina represent one of the most informative sources about the paleoecology of south american megafauna however there are no records of an accurately dated carnivore herbivore taphocenosis this paper presents preliminary results of a recent excavation at the margins of the salado river on sediments attributed to the lujan formation late pleistocene early holocene the fossiliferous strata consist of greenish brown sandy clays deposited in a small paleopond environment that was filled by fluvial sediments exhibiting abundant organic matter gypsum and carbonate concretions fieldwork yielded an association of extinct large sized mammals that include the horse hippidion principale the saber tooth cat smilodon populator at least one adult and one juvenile associated specimen a giant ground sloth megatherium americanum and the glyptodont doedicurus clavicaudatus four 14 c dates were performed on organic matter 12 100 100 14 c bp a femur of s populator 13 400 200 14 c bp and a cervical vertebra of hippidion principale 12 860 120 14 c bp and a pelvis of d clavicaudatus 12 380 190 14 c bp situating the site within 12 500 and 13 500 years approximately remarkably some of the recovered specimens exhibit conspicuous bone modifications furrows pits punctures striations and crenulated margins attributed to the activity of a medium large carnivore the association between bones of herbivore mammals with conspicuous modifications produced by a large carnivore and the presence of cub and adult remains of smilodon link this felid with at least part of the excavated association

a study of the earth s response to surface loading with application to lake bonneville summary analytical solutions are presented for the deformation of a three layered plane earth with elastic viscoelastic rheology and subject to time dependent vertical surface loads the models investigated are 1 elastic or viscoelastic layers overlying an inviscid fluid 2 viscoelastic half space 3 elastic layer over a viscoelastic layer with different boundary conditions at the base of the second layer emphasis is given to the interdependence between the dominant parameters of thickness of the elastic layer the effective viscosity of the viscoelastic layer the change in viscosity with depth and the wavelength and duration of loading the models have been applied to the loading and unloading of the late pleistocene pluvial lake bonneville using the load histories given by crittenden the studies differ from earlier analyses of the bonneville uplift in several important aspects including a rigorous treatment of the horizontal and vertical coupling between the elastic and viscoelastic media the mantle is treated as viscoelastic rather than as a viscous medium the wavelength information has been introduced by using a more realistic model for the load geometry and the effect of the wasatch fault has been considered the models indicate that the two principal parameters defining the rebound are the effective viscosity of the upper mantle and the effective thickness of the elastic layer and that those two parameters can be separated only if both wavelength and amplitude information of the deformation is available at more than one epoch observations of uplift at one epoch are not sufficient to resolve the thickness of the elastic layer and the effective viscosity of the upper mantle but upper bounds for these quantities are well established as 30km and 1 2 10 poise respectively the uplift results do not confirm or deny the possibility of a low viscosity channel in the upper mantle but this can be resolved only if the uplift data are supplemented with observations of the present uplift rate the published rates are controversial but if valid they point to a thin 15 km elastic layer and a relatively narrow 200km low viscosity channel while the models predict uplift near the centre of the former lake they also predict that the peripheral regions should be subsiding appreciably if flow is confined into a shallow channel of low viscosity observations reported by anderson bucknam suggests such behaviour likewise the present rate of uplift changes sign at some distance from the centre of the lake and detailed observations of warping of the shorelines and repeated levelling in the area may yield further constraints on the rebound model copyright