vegetation and climate change since 14 810 14c yr b p in southeastern uruguay and implications for the rise of early formative societies abstract this article presents a combined pollen and phytolith record of a 1 70 m sediment core from the wetlands of india muerta 33 42 s 53 57 w in the lowland pampa grasslands of southeastern uruguay six 14c dates and the pollen and phytolith content of the samples permitted the recognition of four distinct climatic periods between 14 850 14c yr b p and the present the late pleistocene period between ca 14 810 and ca 10 000 14c yr b p was characterized by drier and cooler conditions indicated by the presence of a c3 dominated grassland these conditions prevailed until the onset of the warmer and more humid climate of the holocene around 9450 14c yr b p the early holocene between around 10 000 and 6620 14c yr b p was characterized by the establishment of wetlands in the region as evidenced by the formation of black peat the increase in wetland taxa and the replacement of c3 pooideae by c4 panicoideae grasses during the mid holocene around 6620 14c yr b p began a period of environmental change characterized by drier climatic conditions which resulted in the expansion of halophytic communities in the flat low lying areas of the wetlands of india muerta about 4020 14c yr b p a massive spike of amaranthaceae chenopodiaceae coupled with a radical drop in wetland species indicates another major and more severe period of dryness after ca 4000 14c yr b p a decrease of halophytic species indicates the onset of more humid and stable climatic conditions which characterized the late holocene the findings reported in this article substantially improve our knowledge of the late glacial and holocene climate and vegetation in the region the data provide a detailed record of the timing and severity of mid holocene environmental changes in southeastern south america significantly the mid holocene drying trend coincided with major organizational changes in settlement subsistence and technology of the pre hispanic populations in the region which gave rise to early formative societies this study also represents the first combined pollen and phytolith record for southeastern south america reinforcing the utility of phytoliths as significant indicators of long term grassland dynamics

surface wind measurements in sequal i data from meteorological recorders at 29 37w 24 00w and 14 93w all within one degree latitude of the equator show that a relaxation event in the wind speed occurred nearly simultaneously at all three during late march 1983 during sequal i wind stress subsequently increased by approximately a factor of two the event was not recognizable in the data from a fourth recorder at 6 10n 27 88w

the relationship between emergence of mound builders in se uruguay and climate change inferred from opal phytolith records abstract the strong correspondence between the spatial arrangement of archaeological sites and wetland environments is one of the main axes of the merin lagoon basin archaeology without implying a deterministic simplification from the epistemological point of view the cultural and environmental history of the region hold evidence suggesting a more complex scenario where cultural responses to climate change were not so mechanical or direct as previously argued the relationship between paleoclimate and prehistoric mound builders development between 7 0 and 0 6 ka 14 c bp was studied in se uruguay paleoclimatic data were inferred from the phytolith record of three lagoon sediment cores four paleoclimatic periods were identified according to temperature humidity changes the first period dated at 7 0 4 5 ka 14 c bp was characterized by a warm humid climate between 4 5 and 3 5 ka 14 c bp the onset of a cool dry climate was inferred a hiatus between 3 5 and 2 6 ka 14 c bp was identified in the third period 2 5 1 2 ka 14 c bp similar climate conditions to those of the present were inferred finally the period 1 2 and 0 6 ka 14 c bp was characterized by warmer and wetter conditions than those of the present according to these data it was determined that mounds were first observed during the transition from warm humid to cool dry conditions but the complexity of the mounds increased after the full establishment of cool dry conditions with the inception of warm humid conditions an increased number of mounds was observed in the plains located close to the merin lagoon littoral the mounds showed lower density and height than those recorded for the cool dry period this behavioral modification in mound construction was explained because of the paleoenvironmental change

comments on the paper fibrous clay mineral formation and soil evolution in aridisols of northeastern patagonia argentina by p j bouza m simon j aguilar h del valle and m rostagno

holocene landscape evolution along the andean piedmont bolivian chaco abstract as part of the gran chaco the andean piedmont in eastern bolivia is characterized by a highly seasonal climate and large rainfall variability it receives sediments from small drainage basins in the subandean ranges therefore it should constitute a suitable sedimentary and geomorphic archive for the detection of past environmental changes this study reconstructs holocene landscape evolution along the andean piedmont in the bolivian chaco by combining large scale geomorphological analysis and the investigation of sedimentary sequences fluvial aggradation and channel avulsions are the dominant processes of piedmont construction and were particularly pronounced during the mid holocene this is interpreted to reflect a generally active landscape characterized by reduced forest cover as result of increased aridity widespread soil formation and landscape stability in all investigated catchments start after 4 cal ka bp indicating the onset of wetter conditions and successively denser forest cover in the bolivian chaco renewed sedimentation during the last millennium could imply a shift to more arid environmental conditions probably in relation to larger scale climatic phenomena such as the little ice age lia particularly during the last centuries the fluvial system of the study area has undergone the most profound changes of the entire holocene with sedimentation of very coarse sediments and subsequent incision this reflects important changes both in discharge and sediment supply and might be the result of either shorter term climate changes or increasing human impact

late quaternary marginal marine deposits and palaeoenvironments from northeastern buenos aires province argentina a review the late quaternary marginal marine deposits along eastern argentina southwestern atlantic are reviewed according to our present knowledge in the northeastern coastal area of buenos aires province they have been assigned to a series of transgressions and regressions ranging from the late pliocene to the late quaternary the most widely accepted model is frenguelli s 1957 classical chronostratigraphical scheme of belgranense late pleistocene marine sediments at 3 6 m above m s l and ca 26 000 35 000 14c years bp the querandinense pleistocene holocene estuarine sediments below or at present m s l and the most extensive platense mid holocene marine deposits at 4 5 2 m above m s l dated at ca 8000 1340 14c years bp the restricted belgranense deposits recorded in samborombon bay in magdalena at ca 32 000 bp near mar chiquita at ca 24 900 and 30 500 bp and southwards in bahia blanca at ca 26 000 35 500 bp may belong to an interstadial gonzalez et al 1986 the molluscan composition suggests a marine invasion of the area but not a typical interglacial cycle characterized by euhaline and warm water elements however the oxygen isotope record argues against an interstadial during the interval 34 27 ka and the chronological control for these deposits is very poor suggesting that they most probably have been elevated neotectonically the pleistocene holocene querandinense deposits extensively distributed along the bonaerensian coastal plain and continental shelf ca 11 000 14c years bp with very low faunal diversity abundance of freshwater ostracods and absence of the warm water molluscs characteristic of the holocene ridges indicate low salinity and cool water conditions further dating and isotope analysis of these deposits are required for a better understanding of the chronology of climatic events by the end of the pleistocene in this area and to establish whether or not they could correspond to the younger dryas event of the northern hemisphere platense littoral ridges formed between ca 7600 and 2500 bp extend in several subparallel rows from ca 34 s to ca 39 s they formed either as beaches or possibly as sublittoral bars in samborombon bay where they reach 120 km long 10 30 m wide and up to 5 m thick at 4 5 5 m above m s l or in the supratidal and intertidal zones in mar chiquita at 2 4 m above m s l molluscs and ostracods suggest a brackish marine to marine brackish environment of lower salinity than the modern atlantic littoral during the mid holocene the oceanic waters were mixing with large amounts of antarctic ice melting in the south atlantic isla 1990 probably reducing salinity along the bonaerensian littoral the oldest littoral ridges accumulated during the hypsithermal and in the punta indio area their different geometry and alignment greatest age highest diversity and warmer affinity of the molluscan fauna suggest a short interval of reversal of the atmospheric circulation pattern further research is necessary for a better understanding of the chronology of the marine late quaternary deposits coastal evolution and climatic changes in this area

geochemical evidence of the 8 2 ka event and other holocene environmental changes recorded in paleolagoon sediments southeastern brazil abstract the paleoclimatic record of jureia paleolagoon coastal southeastern brazil includes cyclic and gradual changes with different intensities and frequencies through geological time and it is controlled by astronomical geophysical and geological phenomena these variations are not due to one single cause but they result from the interaction of several factors which act at different temporal and spatial scales here we describe paleoenvironmental evidence regarding climatic and sea level changes from the last 9400 cal yr bp at the jureia paleolagoon one of the main groups of protected south atlantic ecosystems geochemical evidences were used to identify anomalies from multi proxy analyses of a paleolagoon sediment core the anomalies of centennial scale were correlated to climate and transgression regression cycles from the holocene period decadal scale anomalous oscillations in the quaternary paleolagoon sediments occur between 9400 and 7500 cal yr bp correlated with long and short term natural events which generated high sedimentation rates mainly between 8385 and 8375 cal yr bp 10 cm yr our results suggest that a modern day short duration north atlantic climatic event such as the 8 2 ka event could affect the environmental equilibrium in south america and intensify the south american summer monsoon

the glaciogenic origin of the pleistocene calcareous dust in argentina on the basis of field mineralogical textural and geochemical analyses calcareous dust occurs in argentina as layers and pockets closely associated with pleistocene deposits and periglacial features from southernmost patagonia to at least the mendoza precordillera and has been traditionally interpreted as a soil horizon resulting from postdepositional pedogenesis during interglacials detailed field and microscopic observations and sedimentological and geochemical analyses of more than 100 samples collected from lower to upper pleistocene deposits between 51 s and 33 s and from near sea level to 2800 m asl allow us to interpret the dust as synchronous with the host sediment all observations and analyses lead us to conclude that 1 the cryogenic morphology and the chemical signatures of the calcite component show that the dust is glaciogenic 2 the dust was carried by southeasterly antarctic winds and 3 it was deposited over most of southern and central argentina field observations geomorphic evidence and radiocarbon dates suggest that the dust was deposited during several pleistocene glacial episodes

seasonal variation of the gulf of guinea thermal structure data from 6 temperature sections are combined with other sequal focal data along 4 w from the coast 5 n to 3 s to study the seasonal variation of the temperature field in the gulf of guinea during 1983 the equatorial upwelling begins during mid april approximately 1 1 2 months before the climatic mean but has a normal amplitude and duration the upwelling on the coast at abidjan occurs approximately 1 1 2 months after the equatorial upwelling and precedes its climatic mean by 2 weeks at the equator vertical displacements of isotherms within the thermostad are complicated by an anomalous warming of the thermostad in may between the thermostad and the deeper thermocline at 230 m the seasonal isotherm displacement amplitude has a minimum at greater depths it increases again in all respects the fluctuation of the thermal field at the equator is similar to the climatic mean but displaced forward in time by 1 1 2 months

taphonomic processes affecting late quaternary molluscs along the coastal area of buenos aires province argentina southwestern atlantic molluscan concentrations were abundantly deposited along the coastal area of buenos aires province argentina southwestern atlantic by different sea level episodes during the late quaternary most of them represent holocene ridges highly energetic depositional environments subparallel to the modern shore where the main associations preserved are parautochthonous and others mainly autochthonous associations occurring in tidal flats seaward of the ridges or coastal lagoon deposits accumulated behind them low energetic analysis of three physical taphonomic signatures disarticulation fragmentation and abrasion of the molluscan skeletons of four bivalve species tagelus plebeius mactra isabelleana pitar rostratus brachidontes rodriguezi which exhibit different shell shape and thickness conditions habitats and life habits and represent the most widely distributed and palaeoecologically significant taxa along the area of study support the distinction of both palaeoenvironments characterized by low and high taphonomic grades respectively the taphonomic alteration breakage and abrasion of most shells seems to have been a direct consequence of the amount of reworking of the material especially during periods of exposure along an extensive beach zone more than a consequence of transport from a distant original habitat abrasion signatures are the only probably correlated with after death time span shells showing a near modern aspect and original luster being most probably younger than those with chalky or polished appearance the infaunal species analysed typically living on clays and silts or fine sandy bottoms of the infralittoral or intertidal t plebeius m isabelleana are better preserved less fragmented and abraded than shells of the epibyssate b rodriguezi which lives on hard bottoms of the intertidal and supralittoral areas more exposed to highly unstable conditions with longer episodes of exposure although these nearshore concentrations may represent a time averaging of 1000 yr b p taking into account the ecological requirements of the most constant and dominant taxa together with the taphonomic analysis of shells from parautochthonous and autochthonous associations they still represent valid indicators of past marginal marine conditions in the area but are only reliable to establish long term palaeoenvironmental variations

winds at st peter and st paul rocks during the first sequal year wind observations were obtained during the first nine months of the sequal field program from a station maintained at st peter and st paul spp rocks winds are very steady from may to november and very unsteady during april when the intertropical convergence zone itcz crosses the site wind stress components are computed and comparison with 1979 data shows that the onset of the winds occurred one month earlier in 1983 but is of similar amplitude the spectral analysis of the wind shows a significant increase in the variance for periods between 3 5 days and 10 15 days

quaternary fluvial systems of tropics major issues and status of research abstract fluvial systems in tropical regions are not only the lifeline for modern civilizations there but also form an important continental archive for reconstructing quaternary palaeoclimates and palaeoenvironments tropical rivers drain through a variety of geological and geomorphological settings across the world and have been studied for their hydrology sediment transport characteristics facies models stratigraphic development and flood hazards this paper presents an overview of the research on tropical rivers globally and identifies major gaps and unanswered research questions as a part of the igcp 582 on tropical rivers the participants of this project are continuing to tackle many of these issues the collection of papers in this special issue is an outcome of this activity and presents new data on processes and alluvial stratigraphy of the tropical rivers from china india south america and northern australia

a review of late quaternary inland dune systems of south america east of the andes abstract inland dune systems of south america occur in a variety of environmental settings under different climatic conditions from humid subhumid to semiarid arid this contribution provides the state of the art on dune research of inland systems east of the andes tropical northern 10 n 10 s including the dune fields of llanos del orinoco and the amazonian basin are mainly formed from parabolic blowout or deflation basins and linear dunes developed on river floodplains aeolian activity was reported to occur near the late pleistocene holocene boundary in llanos del orinoco over the last 17 ka in the branco river basin and 32 6 ka in the negro river basin both in the amazonian region in general the attributed aeolian origin is controversial the morphology difficult to determine and the chronology still extremely limited and rather debatable in some cases the southern tropical region 10 s 22 s includes aeolian systems situated across the extensive lowlands of the gran chaco and those located in fluvial basins related to the cratonic brazilian shield sao francisco river pantanal and mato grosso the oldest barchanoid barchan ridges at the gran chaco were dated 36 33 ka and subsequent aeolian sand accumulation occurred at 18 ka 14 12 ka 10 9 ka and mid to late holocene times parabolic dunes at the sao francisco river floodplain show an oldest episode generated between 28 and 11 ka followed by eolian activity during the early to mid holocene no chronological data is available for the linear and lunette dunes described at the pantanal linear ridges were distinguished in the upper parana river mato grosso with associated sand accumulation at different intervals of the holocene the last reviewed region is the andean piedmont and western pampas at subtropical eastern south america comprising a large variety of dune morphologies barchan barchanoid linear parabolic transverse and lunette dunes deflation basins and sand sheets luminescence ages suggest the occurrence of aeolian activity intervals during late glacial times and the holocene the general understanding on the dynamic of south american dune systems is hampered by the paucity of chronological information and the representativeness of some dune records whether they reflect local or regional environmental conditions is still a matter of discussion in several areas that need further studies

paleoenvironmental reconstruction of quaternary valley fill successions in summit paleosurfaces of southern sierras pampeanas cordoba province argentina abstract climatic and environmental changes were inferred from the study of late quaternary sequences that represent paleo valleys filling in the summit pampas at the center south of the sierra de comechingones cordoba province argentina a representative sedimentological section p1 was described and sampled and sediments age 14 c granulometric characteristics and organic matter content were determined in the analyzed succession four main stratigraphic units were recognized and interpreted the stratigraphic and pedological characteristics diatom levels presence geomorphological features and altitude conditions of the study area allow us to conclude that 1 valley fill deposits are holocene from 8310 110 yr bp to present days and composed exclusively of loessic loessoid material from surrounding areas 2 the dominant transport mechanism could have been laminar and or hyperconcentrated flows mainly associated with slope evolution processes sheet flow and high density flows 3 isolated swamps with important biological activity were the dominated environments in to the valleys these environments suggest an alternating pedogenic cycles were the most important was the middle late holocene indicating a landscape stabilization tendency 4330 130 yr bp 4 temperate wet climatic conditions and high seasonality characterized middle early holocene hypsithermal period more arid conditions dominated late holocene from 1500 to 1000 bp to the present temperate wet climatic conditions occurred and this period was interrupted by the little ice age 5 in the mountains both in wet and dry periods the rainfalls were probably higher as well as the temperature must be lower than the surrounding plain especially in winters perhaps reaching periglacial conditions during more arid periods

patagonian dust in antarctica abstract recent advances in the knowledge of antarctica have demonstrated that the atmospheric dust trapped in the west antarctica glaciers during is2 4 and 6 has a patagonian origin a climatic scenario is developed in this article in order to explain such a process patagonia is unique as it is the only sizable land mass located in the belt of westerlies in the southern hemisphere the westerlies are formed by cyclonic structures which normally elevate the air masses and aerosols to the upper troposphere there the phenomena of mass compensation occur with the antarctic anticyclone which as any anticyclone sinks air and aerosols to the surface large quantities of loose surficial sediments and atmospheric dust are typical of patagonia and can easily be picked up by the wind according to this scenario which is based on the general atmospheric circulation australia and southern africa are hindered in providing aerosols to antarctica because those territories are located in the subtropical high pressure belt

basin wide thermocline displacements along the equator of the atlantic in 1983 observations from five inverted echo sounders deployed between 10 w and 38 30 w along the equator during the spring and summer of 1983 are presented the annual development of the eastward acting zonal pressure gradient is described and related to the observed zonal wind stress the minimum and maximum basin wide gradients are observed in mid april and in the latter part of july respectively the upwelling season in the gulf of guinea is identified at the easternmost sounder during may june and july pulses in thermocline displacement first deepening then shallowing propagate eastward across the basin at speeds of 2 8 and 2 1 m s 1 they arrive at 10 w at the onset and just after the cessation of the seasonal upwelling at that location their initialization is related to relaxation periods in the zonal wind for four of the last five years the time of the pulse preceding the upwelling is tentatively identified

sea level fluctuations during the last 8600 years in the de la plata river argentina abstract the de la plata river considered from the geomorphologic point of view can be defined as a unit composed of a set of features genetically linked together which were formed as a result of the post last glacial maximum transgression regional stratigraphical morphological and topographical studies carried out in the de la plata river and surrounding areas of eastern argentina determined that the holocene evolution of the area was the result of processes resulting from relative sea level fluctuations in order to date the different stages of evolution a curve of sea level fluctuations was constructed after a selective choosing of samples with reliable ages supported by field geological evidence the curve shows a rapid initial transgressive event and the maximum highstand of mean sea level at 6000 yr bp at a relative elevation of 6 5 m above present the following regressive event was characterized by a discontinuous fall in sea level geomorphological evidence suggests that sea level fall could have experienced minor changes in rate as well as moments of stability that are not shown by the curve short interruptions in the geological record can be explained either by the same minor changes in sea level or by a change in wave regime correlation of transgressive regressive events with paleoclimatic and biogeographic information proposed that holocene sea level fluctuations in the de la plata river were mainly controlled by climatic changes

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

crustal conductivity anomalies in central sweden and southwestern finland summary lateral conductivity variations in the earth s crust in central sweden and southwestern finland were mapped using magnetic field variations recorded simultaneously by 29 gough reitzel type magnetometers in summer 1985 three events of 3 or 4 h were digitized and analysed to obtain single station induction arrows and interstation horizontal field transfer functions the 1985 magnetometer array covered part of the bear baltic electromagnetic array research array operated in 1998 on the fennoscandian shield results from eight bear sites are used to control and to supplement the 1985 array results the conductivity of the crust was found to increase in the western part of the array in western central sweden crustal conductors were found both in the western part of the array and in the baltic sea area in the eastern part of the array 3 d modelling based on the induction arrows revealed a major conductivity anomaly in the middle and lower crust under the archipelago of southwest finland

drifting buoy trajectories in the atlantic north equatorial countercurrent during 1983 during 1983 21 freely drifting buoys measured trajectories in the vicinity of the atlantic north equatorial countercurrent these provide a visualization of the flow field and the first direct measure of the seasonal variation of the velocity during a single year from february to may the buoys moved gradually westward from june to december the buoys moved swiftly eastward with maximum speeds of 90 cm sec 1 located in the western atlantic the seasonal variation in current was similar to that measured by historical ship drifts except that the onset of the countercurrent in 1983 was about a month earlier than usual of the buoys that drifted eastward in the countercurrent three went into the north equatorial current two into the south equatorial current and seven into the guinea current

quaternary of the pantanal west central brazil abstract the pantanal is a tectonic depression located at the left margin of the upper paraguay river the paraguay is the trunk river of an alluvial depositional tract composed by several large marginal alluvial fans the taquari fan being the largest one the present landscape is a complex tropical wetland characterized by month long floods every year with geomorphic features derived from the present conditions and others inherited from successive pleistocene and holocene climates some areas containing ponds are landscape relicts generated by eolian deflation during the last glacial maximum many ponds closed depressions isolated from the superficial waters by vegetated crescent ridges of fine sands were interpreted as salt pans bordered by lunette sand dunes initiation of the modern wetland has occurred during the pleistocene holocene transition with the change to a more humid climate and the individualization of lacustrine systems active tectonics has been playing an important role in the development of the pantanal landscape nowadays the paraguay river meanders in a large flood plain with extensive swamp surfaces being structurally constrained by faults in the west border of the basin sedimentation within the pantanal wetland is also affected by tectonic activity especially along faults associated with the transbrasiliano lineament

a climatic model for southwestern amazonia in last glacial times abstract multidisciplinary data indicate that southwestern amazonia was a savanna environment during last glacial times interpretation of biogeographical and geological records as compared with the present climatic conditions permit us to postulate a dry season more pronounced and prolonged than the present one dry northerly winds vinculated with southern bemisphere trade winds dominated during the dry season and the southerly cold air masses locally termed friagem or surazo would have been more frequent and intensive than today

modes of variability of the 1983 thermocline signal acoustic travel time series from an array of nine inverted echo sounders deployed along the equator and in the north equatorial countercurrent necc are decomposed into empirical orthogonal functions eof to analyze the dominant frequencies of oscillations in the thermocline field at low frequencies the first eof mode describes the seasonal signal shows the reversal of the necc and separates the behavior of the gulf of guinea from the rest of the basin the second eof mode shows rapid changes in the depth of the thermocline in short periods of time on the equator at 10 w and at 3 n 38 30 w comparison with a simulation model by philander and pacanowski shows that the theoretical time series behave in a similar way at these two locations at high frequencies the modes show oscillations in the inertia gravity wave band

climatic events during the late pleistocene and holocene in the upper parana river correlation with ne argentina and south central brazil most quaternary studies in brazil are restricted to the atlantic coast and are mainly based on coastal morphology and sea level changes whereas research on inland areas is largely unexplored the study area lies along the parana river state of parana brazil at 22 43 s latitude and 53 10 w longitude where the river is as yet undammed paleoclimatological data were obtained from 10 vibro cores and 15 motor auger holes sedimentological and pollen analyses plus tl and 14c dating were used to establish the following evolutionary history of late pleistocene and holocene climates first drier episode 40 000 8000 bp first wetter episode 8000 3500 bp second drier episode 3500 1500 bp second present wet episode 1500 bp present full size table table options view in workspace download as csv climatic intervals are in agreement with prior studies made in southern brazil and in northeastern argentina

micromorphology and osl dating of the basal part of the loess paleosol sequence at la mesada in tucuman province northwest argentina abstract the basal part of the 42 m loess paleosol sequence at la mesada near tucuman in nw argentina reported by zinck and sayago j s am earth sci 12 1999 293 quat int 78 2001 11 is re examined here using a combination of micromorphology and optically stimulated luminescence osl dating depth distributions of microstructure clay coatings and secondary carbonate features enable the development of this part of the sequence to be reconstructed in terms of a series of pedosedimentary stages stage one involved accretionary pedogenesis with loess being modified by bioturbation and possibly leaching processes as it accumulated stage two was marked by a reduction in loess deposition and establishment of a relatively stable surface at which bioturbation leaching and clay translocation processes were dominant stage three represented a return to accretionary pedogenesis followed by establishment of a new relatively stable surface and vertical leaching and clay translocation processes with some welding of secondary carbonate features on the underlying older soil the three osl dates ranging from ca 150 to 195 ka bp suggest that the whole sequence is significantly older than indicated by the radiocarbon chronology ka bp reported in the works of zink and sayago cited above however assuming that the pedosedimentary changes reflect alternating periods of dry cool and moist warm conditions as suggested in the works of zink and sayago cited above the succession still potentially provides the highest resolution proxy climatic record so far reported in argentina

paleoenvironmental insights into the quaternary evolution of the southern brazilian coast based on fossil and modern diatom assemblages abstract diatom assemblages provide a strong basis for detailed interpretations of paleoceanography and diatom paleoecology of the southern brazilian coast nine cores obtained in the coastal plain shelf and continental slope and thirteen surface sediment samples of the patos lagoon provide an excellent opportunity to use a paleoecological approach to study detailed quaternary environmental changes in the southern coasts in the interval studied the basin sedimentation occurred in a marine dominated environment related to humid periods prior to 43 500 year bp abundant diatom taxa are highlighted as excellent indicators of environmental changes including salinity sediment composition and transport by incised valleys providing additional insights into coastal evolution paralia sulcata is dominant in all analyzed cores occurring continuously from late pleistocene to holocene but is rare in the modern sediments non marine diatom assemblages occurred continuously but in very low densities in periods before and after the last glacial maximum abrupt changes in sedimentation have been recorded related to shifts from marine dominated to shallow estuarine deposition during the holocene sea level rose rapidly which is responsible for the progressive coastal flooding prior to 8420 7930 year bp the holocene barrier system became more continuous and patos lagoon developed consequently the secondary former inlets from the jacui and camaqua rivers are closed and only one single inlet rio grande channel remains active the spatial distribution of modern assemblages is controlled by salinity gradient wind action and climate forcing the bottom sediments of patos lagoon are dominated by freshwater diatom aulacoseira veraluciae aulacoseira sp 2 and marine estuarine species of cyclotella litoralis

variation in isotopic composition c o and sr of holocene mactra isabelleana bivalvia from the coast of buenos aires province argentina the 13c 12c and 18o 16o ratios of carbonate incremental growth bands and whole shells of the bivalve mactra isabelleana collected from holocene including modern littoral deposits along the coastal area of buenos aires province close to the rio de la plata river reflect different environments which stayed relatively constant throughout the holocene differences in 13c 12c and 18o 16o ratios along the growth axis of two shells from the modern estuarine and marine situations constrain the differences between the two types of environment the data show a marked cyclicity from both sites but the increase in river flow during the winter is reflected by very low d18o and d13c values in the estuarine shell in contrast the shell from the marine site has fewer cycles with lower amplitude which are consistent with seasonal temperature variations away from the influence of river input the differences in the isotopic cyclic variations between the marine and estuarine sites average out in the whole shell analys

diet and habitat of toxodont megaherbivores mammalia notoungulata from the late quaternary of south and central america abstract the toxodont megaherbivores toxodon and mixotoxodon were endemic to south and central america during the late quaternary isotopic signatures of 47 toxodont teeth were analyzed to reconstruct diet and ancient habitat tooth enamel carbon isotope data from six regions of south and central america indicate significant differences in toxodont diet and local vegetation during the late quaternary toxodonts ranged ecologically from c3 forest browsers in the amazon mean 13c 13 4 to mixed c3 grazers and or browsers living either in c3 grasslands or mixed c3 forested and grassland habitats in honduras mean 13c 9 3 buenos aires province argentina 13c 8 7 and bahia brazil mean 13c 8 6 to predominantly c4 grazers in northern argentina 13c 4 4 to specialized c4 grazers in the chaco of bolivia 13c 0 1 although these toxodonts had very high crowned teeth classically interpreted for grazing the isotopic data indicate that these megaherbivores had the evolutionary capacity to feed on a variety of dominant local vegetation in the ancient amazon region carbon isotope data for the toxodonts indicate a c3 based tropical rainforest habitat with no evidence for grasslands as would be predicted from the neotropical forest refugia hypothesis

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

letter to the editorcomment on a prieto s 1996 paper late quaternary vegetational and climatic changes in the pampa grassland of argentina

late pleistocene and holocene landscape evolution and environmental change in the lake sulivan area falkland islands south atlantic abstract late pleistocene and holocene landscape evolution around lake sulivan falkland islands south atlantic is assessed using morphological stratigraphical palaeoecological and 14c dating evidence data indicate a complex landscape history involving frost related activity ventifact development growth and degradation of peat terrain expansion and contraction of lakes and the development stabilisation and subsequent reworking of alluvial and aeolian sand bodies late pleistocene cold conditions favoured production of sandy diamicts on hillslopes and ventifacts on outcrops and blockfield clasts lateglacial climatic amelioration was marked by early growth of peat ca 16 500 cal yr bp since then peat accumulation changes in lake areas and depths and alluvial and or aeolian sand deposition have been recurring and dominant features of landscape change which throughout the late pleistocene and early holocene may have been a response to near synchronous climatic fluctuations along the south america antarctica axis marked landscape changes around lake sulivan coincided with intervals of glacier readvance in south america and south georgia at ca 3200 1900 and 700 100 cal yr bp with lower lake levels and aeolian sand transport from exposed lakebeds and shorelines implying greater aridity in the falklands than at present

anthropogenic geomorphic change as a potential generator of renewable geologic resources in the humid pampa of argentina abstract the nature and amount of sediments in various lakes intermittent swampy areas and river sectors in the humid pampa buenos aires province argentina were analysed the aim was to determine whether recent sediments in such environments could serve as an alternative resource to the brick industry for minimising the current high environmental impact of soil mining sediment sequences were obtained and the thicknesses of the upper sediment layers corresponding to the suballuvial approx 1400 700 years bp and alluvial approx 250 years bp to present were determined sediment samples were collected and analysed for grain size and atterberg limits suitable sediments were then selected to determine the optimal brick materials and their technical properties similarly control bricks were prepared with ceramic pastes of local industries the results show that the quality of the former is similar or superior to that of the latter the initial estimates of the available resource indicate a long term supply for the industry estimates of the current sedimentation rates indicate that resource renewal might occur at a rate comparable to current consumption the sedimentation rates have increased significantly in the past two centuries more so in the past few decades the anthropocene with increasing human modification of geomorphic processes if the results presented here are confirmed a highly sustainable model can be implemented in the brick industry

last glacial maximum and hypsithermal in the southern hemisphere abstract a synthesis of the two contrasting climates of the upper quaternary in the southern hemisphere based on continental geological indicators suggests that the antarctic anticyclone is the most important climatic system in the hemisphere according to glaciological and oceanographic studies the antarctic anticyclone covered an area in the lgm that was double of that during the hypsithermal this phenomenon produced a consequent shifting of the climatic belts by approximately 10 of latitude to the north in the lgm and a migration to the south during the hypsithermal oceanic and continental anticyclones were active systems in shaping climates in south america australasia and southern africa these structures were enhanced throughout the lgm provoking generalized droughts and weakened at the hypsithermal permitting more rains in tropical latitudes the results presented here were obtained by integrating our own results with regional and continental syntheses of other authors the paleoclimatic proposals are amply validated by more than 1500 dates in the four continents geological and geomorphological features such as dunefields paleosols fluvial terraces and lake levels were the main tools employed in the work

isotopic constraints on the source of argentinian loess with implications for atmospheric circulation and the provenance of antarctic dust during recent glacial maxima we present rare earth element ree and sr nd isotopic data for argentinian loess 28 38 s with two aims 1 to examine the source regions of argentinian loess and the constraints that these put on palaeo wind directions 2 to further investigate the source of antarctic ice core dust and to test the hypothesis that some of it could be derived from a region to the north of patagonia into which the dry dusty westerly dominated patagonian climate expanded during quaternary glacial maxima sr nd isotopic data for argentinian loess from north of 37 s are distinct from patagonian loess compositions in that they have more radiogenic sr 87sr 86sr 0 7059 0 7123 and less radiogenic nd nd 0 8 to 6 4 ree patterns and sr nd isotopic values are relatively homogeneous for multiple samples taken from single loess sections but show significant differences between sections in general there is a northward change from patagonia like ree patterns and isotopic values away from volcanogenic signatures and towards those that are more like the continental crust the latitudinal nd isotopic pattern is remarkably similar to that for andean volcanic rocks and suggests derivation of loess from the andes by more or less direct westerly transport for loess sections in the north the data imply a contribution from palaeozoic gneisses to the northwest in the chilean altiplano sr nd data for extra patagonian argentinian loess north of 37 s do not support a significant role for this source region in supplying dust to antarctica at the last glacial maximum this conclusion contrasts with previous studies that suggest a significant northward shift in the climatic belts and in particular the westerlies and the antarctic polar front during quaternary glacial maxima very systematic relations between the sr nd isotopic composition of loess and their andean source highlights shifts in the sr isotopic composition of loess to more radiogenic values and strongly suggests that the slight offset between patagonian loess and ice core dust identified by previous workers is due to grain size differentiation effects

the nature distribution and formation of pans in arid zones abstract pans closed depressions are a widespread feature of many of the world s drylands by using literature survey air photographs topographic maps orbital photographs and imagery combined with field work it is possible to describe the major areas where these features occur their distribution is controlled to a great extent by the availability of susceptible surfaces they also develop in certain particular environmental settings palaeolacustrine basins palaeodrainages interdunes and on coastal plains many of the pans have a distinctive morphology while on their lee sides they may have lunette dunes many processes combine to create and maintain pans and these can be considered in a general model which has certain key elements the first of these is that the area should not be one where fluvial processes are fully integrated it should also not be one where aeolian accumulation is such as to infill any irregularities in the land s surface if these two predisposing conditions are fulfilled then under dryland conditions if susceptible surfaces are present there are various circumstances that may lead to hollow development and enlargement although in some cases such processes as solution suffosion and animal activities may play a role we believe that the predominant reason why pans have the characteristics that they do including their shapes lunettes alignments etc is that they result from the operation of the twin processes of salt weathering and aeolian deflation

reply to the letter to the editor on late quaternary vegetational and climatic changes in the pampa grassland of argentina by m iriondo

changes in the atmospheric ch4 gradient between greenland and antarctica during the holocene high resolution records of atmospheric methane over the last 11 500 years have been obtained from two antarctic ice cores d47 and byrd and a greenland core greenland ice core project these cores show similar trapping conditions for trace gases in the ice combined with a comparable sampling resolution this together with a good relative chronology provided by unequivocal ch 4 features allows a direct comparison of the synchronized greenland and antarctic records and it reveals significant changes in the interpolar difference of ch 4 mixing ratio with time on the average over the full holocene records we find an interpolar difference of 44 7 ppbv a minimum difference of 33 7 ppbv is observed from 7 to 5 kyr b p whereas the maximum gradient 50 3 ppbv took place from 5 to 2 5 kyr b p a gradient of 44 4 ppbv is observed during the early holocene 11 5 to 9 5 kyr b p we use a three box model to translate the measured differences into quantitative contributions of methane sources in the tropics and the middle to high latitudes of the northern hemisphere the model results support the previous interpretation that past natural ch 4 sources mainly lay in tropical regions but it also suggests that boreal regions provided a significant contribution to the ch 4 hudget especially at the start of the holocene the growing extent of peat bogs in boreal regions would also have counterbalanced the drying of the tropics over the second half of the holocene finally our model results suggest a large source increase in tropical regions from the late holocene to the last millennium which may partly be caused by anthropogenic emissions

a late holocene 2600 bp glacial advance in the south central andes 29 s northern chile radiocarbon dates and relative ages of soils on moraines provide evidence of a substantial late holocene 2600 bp readvance of glaciers in the western part of the arid andes at 29 s the glacier in the encierro valley northern chile reached about 60 of its late pleistocene extent the equilibrium line altitude ela during this advance remained between 4250 and 4500 m which is above the modern 0 c isotherm of the free atmosphere during summer it is suggested that given the specific climate at high elevations in the extremely arid andes a substantial increase in effective moisture is sufficient to explain this advance whereas no significant temperature depression was needed this compares favourably with the regional late holocene climatic history when lake levels were higher and effective moisture greater than today between 3000 and 1800 bp judging from regional palaeodata we suggest that this late holocene humid spell was most likely related to increased westerly moisture sources from the pacific

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

late quaternary paleosol sediment sequences and landscape evolution along the andean piedmont bolivian chaco abstract as part of the bolivian chaco the andean piedmont is situated in the transitional zone between the tropical humid climate regime to the north amazonia and the subtropical semiarid regime to the south gran chaco in this zone which is sensitive to climatic changes late quaternary landscape and climate history was reconstructed based on detailed sedimentological pedological and geochemical analyses of paleosol sediment sequences along the andean piedmont the determination of characteristic lithofacies allowed correlation between the investigated profiles and their interpretation in terms of paleoenvironmental conditions i e geomorphic stability soil formation or geomorphic activity fluvial or fluvio aeolian sedimentation erosion etc coarse fluvial deposits gravels sands form the base of the investigated profiles and have likely been deposited in a braided river environment with low vegetation cover and intensive discharge events before and during the last glacial maximum well developed paleosols formed in the upper part of these sediments during the lateglacial documenting geomorphic stability high vegetation cover and wet conditions overlying palustrine sediments dated to the early holocene indicate a dramatic change in seasonality i e increased winter precipitation and decreased summer precipitation subsequent erosion followed by accumulation of fluvial and fluvio aeolian sands points to much drier conditions during the mid holocene since 2 9 cal ka bp soil formation indicates a return to geomorphic stability and wetter conditions characteristic of the present day climate interrupted by short arid intervals

southern westerlies during the holocene deduced from geomorphological and pedological studies in the norte chico northern chile 27 33 s abstract in the norte chico precipitation is controlled by atmospheric perturbations and frontal systems associated with the belt of westerly wind circulation the holocene climatic history of the region can be reconstructed from geomorphic and pedologic investigations in comparison to modern climatic conditions increased influence of the westerlies is deduced for the time periods prior to 7300 yr b p between 5000 3700 yr b p 3000 1800 yr b p and around 270 yr b p it remains uncertain whether the increased influence of the westerlies was the consequence of northward shifts of the westerlies as a whole of a meridional expansion of the westerly belt or only of intensity changes of the atmospheric dynamics almost synchronous with changes in the westerlies the coastal fogs in the region were apparently reduced or the fog belt in the coastal range was elevated probably due to a weakened upwelling and warmer water conditions in the eastern pacific in general the climate of the norte chico has become more variable since 5000 yr b p especially since 3000 yr b p with frequent changes of the geoecological conditions

late quaternary climate history of the bolivian altiplano abstract sediment cores and outcrops from the bolivian altiplano are used to interpret late pleistocene holocene paleoclimates and lake evolution based on a multi proxy approach ostracod content palynology sedimentology and radiocarbon dating despite the different sensitivity of the lacustrine basins to environmental changes interpreting the records in terms of paleohydrology climate and more especially timing of events is difficult notwithstanding these problems lake titicaca in the north and lake pocoyu lake poopo salars of coipasa and uyuni in the south reveal a similar evolution in general trends through the course of time during the minchin phase until 30 26 kyr bp the altiplano was wetter than present the last glacial maximum ca 26 14 kyr bp marked by cooler drier conditions was followed by a return to a wetter climate interrupted by short arid events between ca 14 and 10 5 kyr bp another cycle of aridity between 10 5 and 8 kyr bp took place abruptly just after the humid tauca phase the mid holocene is interpreted as climatically unstable with an alternation of humid and dry episodes since 3 9 kyr bp wetter conditions have persisted and intensified to the present the data are interpreted in terms of changes in inter tropical convergence zone extension summer precipitation and in polar air mass intensity winter precipitation

seasonal variations of the subsurface thermal structure in the gulf of guinea abstract the subsurface thermal structure in the gulf of guinea is analyzed using the historical hydrographic data file of particular interest is the rapid vertical displacement of the thermal structure from the warm march may season to the cold july august season this displacement field is trapped at the equator with vertical and meridional scales appropriate to a second or third mode baroclinic kelvin wave it has a maximum amplitude between 4 w and 10 w with a node near 2 e south of 4 s no seasonal displacement of the thermocline underlies the seasonal variation in the sea surface temperature along the eastern boundary north of the equator to 3 e the displacement field vanishes thus the coastal upwelling associated with a vertical displacement tightly bound to the northern coast from 2 e to 10 w appears detached from the equatorial regime

the mar chiquita lake an indicator of intraplate deformation in the central plain of argentina abstract the mar chiquita saline lake is located in the lowest part of one of the largest endorreic saline basins of south america with a surface area of 6000 6500 km2 the lake is located in a tectonic depression with an asymmetric cross section the sierras pampeanas foothills with altitudes near 1500 m are to the west and a 100 m topographic high san guillermo high is to the east whose western border is bounded by a buried middle pleistocene fault the tostado selva fault the main tributary of the lake is the dulce river which flows from north to south the southward flow of the river was impeded by an obstacle that closed the dulce valley generating the mar chiquita lake the megafans of the primero segundo and tercero rivers deposited a large amount of sediment against the faulted border of the san guillermo high generating an obstacle that impeded the normal flow of the rivers and diverted the dulce and the salado rivers to their present positions precise data concerning the age of the impounding of mar chiquita does not exist but lacustrine conditions are undoubtedly younger than the uplift of the san guillermo high which occurred in the middle pleistocene the well preserved dry valley of the dulce river located southward of mar chiquita is still visible in satellite images and confirms the youth of the impounding the observations introduced in this paper allow us to understand the origin of a significant feature of the central plains of south america the generation of mar chiquita lake and upstream wetlands produced a pronounced environmental change in the arid chaco pampeana plain which favored human life by introducing changes in vegetation and fauna

historical climatic extremes as indicators for typical scenarios of holocene climatic periods in the pampean plain abstract the holocene has been marked by two different climatic periods in the pampas one 8 5 3 5 kyr b p characterized by temperatures and precipitation higher than today and the other 3 5 1 4 kyr b p semiarid generating parabolic dunes the humid period produced intense pedogenesis down to 40 lat s and a mobilization of iron oxides to 30 lat s which means tropical climate with temperatures above 20 c and precipitation higher than 2000 mm yr sand deflation and development of parabolic dunes occur under climates with 300 400 mm yr this contribution is based on geological and physical proxies typical weather scenarios of those climates are sporadically reproduced today during extremely humid or dry periods on that basis meteorological parameters of years beyond the thresholds of 2000 and 400 mm yr were processed dry periods were characterized by a large thermal amplitude frosts and stronger winds reproducing the continental anticyclonic circulation of the late holocene humid extremes were warmer than normal lower thermal amplitude with rains produced by local convection processes typical scenarios with precipitation above 2000 mm yr are 21 c mean temperature ca 1 c higher than the present record 27 c maximal annual temperatures 16 c minimal annual temperatures more than 1 c higher than the present as a consequence thermal amplitude was smaller than today virtually without frosts characteristic parameters of the late holocene were mean annual precipitation ca 350 mm yr 15 c mean temperature 22 c maximal annual temperatures 8 c minimal annual temperatures showing significative shifts in the dry season lower than normal monthly maximal and mean temperatures higher than today the result is a thermal amplitude larger than today probably higher quantity of frosts per year stronger winds and lower relative air humidity in comparison with the humid climate extreme complete the scenario

paleoenvironments in the paso otero locality during late pleistocene holocene pampean region argentina an interdisciplinary approach the recently excavated paso otero 4 site is located within the paso otero archaeological locality necochea district pampean region the site is characterized by the presence of a great variety of archaeological materials and a rich and complex stratigraphic record which allows use of an interdisciplinary approach the objective of this paper is to present the results of the geoarchaeological palynological phytolith and diatom analyses in order to reconstruct the paleoenvironmental conditions for the final late pleistocene through holocene these lines of research allow inference of paleoenvironmental and palaeoclimatic conditions on both local and regional scales at the site the rio salado member unit 2 where the archaeological deposit is buried underwent sudden and fluctuating changes in water availability conditions although the setting was basically characterized by calm depositional processes within and along the margins of interconnected lagoons paleoclimatic reconstructions indicate the existence of abrupt climatic changes throughout the sequence early holocene human occupations took place under regionally temperate warm climates while the end of the middle holocene witnessed cold and arid or cold temperate climates in general terms paleoenvironmental conditions inferred for paso otero 4 are in accordance with regional reconstructions proposed for the pampas

climatic variations in the argentine plains during the last 18 000 years the last deglacial hemicycle was characterized by a general increase in temperature and precipitation in the region with a few significant departures from this general trend the present ne sw climatic gradient was maintained throughout the entire period except in the upper holocene the following sequence of events is apparent if the present climate is taken as a reference base 1 a 18 000 8500 yr b p arid and cool with aeolian sand ad loess deposition patagonian fauna climatic isolines temperature precipitation etc were located some 750 km northeast of their present positions 2 b 8500 3500 yr b p humid subtropical with brazilian fauna pedogenesis and fluvial dynamics climatic limits migrated about 800 900 km southwest of their former positions 3 c 3500 1000 yr b p dry subtropical aeolian dynamics the normal latitudinal climatic gradient was interrupted by the occurrence of an anticyclonic centre which stabilized the climate over an area of some 1 600 000 km2 4 d 1000 yr b p little ice age climate was similar to the present one over much of the plains but the northeastern extremity was warmer 5 e little ice age climatic deterioration in the southern belt was characterized by generalized aeolian activity and migration of isolines more than 150 km to the northeast in that area 6 f present climate 19th and 20th centuries subtropical humid in the east and dry in the west

late pleistocene and holocene palaeoenvironments in golfo san jorge patagonia molluscan evidence palaeoenvironments suggested by molluscan assemblages preserved in late pleistocene and holocene raised marine terraces along the golfo san jorge central patagonia coastline argentina are reviewed in this work a better precision of the stratigraphical and geographical ranges of the molluscs through their systematic review shows that all taxa have living representatives mostly within the argentine and or magellanean zoogeographical provinces in general no significant taxonomic appearances or disappearances are evident along this coastal area since the last pleistocene highstand reinforcing that the modern palaeoceanographical pattern dominated by the cool malvinas falkland current has not been substantially modified and that the magellanean malacological province has lasted at least since the pleistocene although with varying boundaries the so called marine terrace v recently assigned to the last interglacial highstand oxygen isotope substage 5e showed in this area a molluscan assemblage characterised by very low diversity dominantly large shells of mulinia edulis and by cold water taxa in disagreement with higher sea surface temperatures within oxygen isotope substage 5e last interglacial maximum and suggesting softer substrates than in the modern adjacent littoral zone

propagation of the seasonal upwelling in the eastern equatorial atlantic abstract several mechanisms have been proposed to explain the coastal and equatorial upwelling in the eastern atlantic guinea gulf the most controversial is the mechanism of remote wind forcing in the western equatorial atlantic suggested by moore et al 1978 most of the possible explanations for the upwelling and their relative importance are discussed in view of recent observations detailed analysis of daily sea surface temperature sst collected at 16 coastal stations along the northern coast of the guinea gulf reveals that the upwelling event propagates westward along this coast at a mean speed of 0 7 m s 1 similar analysis of historical monthly mean sst data shows that the coastal upwelling event propagates poleward from 1 s to at least 13 s at the same phase speed furthermore the northern hemisphere and southern hemisphere coastal upwelling signals seem to start at the same time from the equator the same kind of analysis applied to hydrographic data from a station situated 41 km off abid

feedback between aeolian dust climate and atmospheric co2 in glacial time 1 enhanced aeolian supply of iron to the biota of the southern ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric co2 declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in co2 but the reasons behind the dust decline itself are as yet unknown here we show that the dust record from the vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of patagonian sources of aeolian material and the efficiency with which it is transported through the atmosphere we suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that in turn climate is responsive to the dust due to iron fertilization this positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states characterized by distinct co2 concentrations recognition of this glacial subcycle may help to account for the timing and sequence of events at the terminations

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

holocene 4 5 1 7 cal kyr bp paleoenvironmental conditions in central argentina inferred from entire shell and intra shell stable isotope composition of terrestrial gastropods the isotopic fingerprint of terrestrial gastropods has been increasingly used as a credible natural paleoenvironmental archive most published work has used this proxy at tropical and temperate latitudes of the northern hemisphere and focused on entire shell analysis the present study provides entire shell and intra shell isotopic profiles to infer average and seasonal late holocene environmental conditions in central argentina 30 s shells of plagiodontes daedaleus gastropoda odontostomidae were retrieved from the alero deodoro roca sector b site one of the few archaeological sites in central argentina rich in shells collected by pre hispanic hunter gatherer groups ancient entire shells exhibited values that were 2 5 higher in 13c and 1 8 higher in 18o than modern individuals pointing to higher abundance of c4 plants and overall drier conditions lower relative humidity and or higher rain 18o during 4 5 1 7 cal kyr bp than today in agreement with published regional proxies intra shel

geomorphic expression of the southern central andes forebulge 37 s argentina abstract we present a geomorphologic analysis of an east west transect located east of the southern andes of argentina 37 s we observe a succession of zones that underwent erosion and deposition during the pleistocene if the proximal andean foothills are incised a proximal depozone receives sediments feeding the megafan of the rio colorado on the chadileuvu plain more distally the abandoned palaeo valleys and bending of the valley floors reflect a localized uplift further to the east another depozone corresponds to the pampa deprimida lowland this pattern is consistent with the presence of a classical flexural geometry of the lithosphere the distal uplift of the foreland corresponds in terms of location length 150 km and amplitude 240 m to the andean forebulge modelled by a geophysical approach in this study we identify the morphological imprint of this bulge and show its effect on the fluvial activity

paleoenvironments and faunal extinctions analysis of the archaeological assemblages at the paso otero locality argentina during the late pleistocene early holocene abstract paso otero archaeological locality pampean region argentina has provided detailed paleoenvironmental reconstructions and faunal assemblages from the pleistocene holocene transition and early holocene this time span is represented by paso otero 5 c 10 450 10 200 14 c bp 12 593 11 353 cal bp and paso otero 4 c 9900 7700 14 c bp 10 667 7982 cal bp palaeoenvironmental studies have been carried out on the basis of data derived from geoarchaeological sedimentary palynofacies pollen diatoms and silica microremains unlike other areas of the pampean region bone assemblages from paso otero include high representation and diversity of taxa in this paper paleoenvironmental reconstructions and faunal assemblages from both sites are integrated in order to discuss the consistency of the inferences derived from the two lines of evidence its contribution to the paleoecological scenarios and to faunal extinctions it is suggested that not only hunting pressure but the mere presence of humans was a factor in ecosystem disturbance and is the most parsimonious explanation for the process of extinction

late holocene glacier variations in the cordillera darwin tierra del fuego chile radiocarbon dates obtained on peat and macrofossils sampled both proximally and distally of a terminal moraine in the bahia pia area cordillera darwin tierra del fuego chile indicate that the glacier system reached maxima sometime before 3060 bp prior to 940 bp and between 940 bp and 675 bp the glacier retreated 4 km to a pinning point position where it dammed a lake before 2605 80 bp between 1950 75 and 1790 75 bp and some time shortly after 1790 75 bp by 250 bp the glacier system had receded more than 15 km towards the fjord head and probably reached close to its present position thus we obtained no evidence from bahia pia of any glacial advance occurring during the global little ice age maximum seventeenth to nineteenth centuries

bone diagenesis and taphonomic history of the paso otero 1 bone bed pampas of argentina samples of guanaco bone from an archaeological site in the pampas of argentina have been analysed to understand the diagenetic profile of the bone assemblages that characterized the taphonomic history of the site two archaeological occupations of paso otero 1 were investigated encompassing similar landscape settings climates and depositional environments the time span is a c 2000 year period from c 4800 to 2800 years bp a total of 30 bone samples taken from both occupations were used to provide a preliminary characterization of the diagenetic pathways at the site the parameters investigated provide a comprehensive account of how both mineral hydroxyapatite and bone protein collagen have been altered in order to compare the two bone assemblages in terms of their diagenetic parameters multivariate analyses were conducted results indicate two different diagenetic profiles in the site n being one of the variables that accounts for most of the variation in paso otero 1 the diagenetic analyses indicate that protein is less preserved in the bone assemblage from the middle stable landscape alternative interpretations of the diagenetic profiles are discussed in light of the taphonomic history of the site and palaeoenvironmental information of the region one hypothesis stresses the importance of the role of climate in defining the different diagenetic pathways and the other the continued action of the combined diagenetic factors along time as the main explanation for the variability in the state of preservation of the bones in paso otero 1

evidence for late quaternary aeolian activity in the roraima guyana region abstract new evidence is provided for the aeolian origin of the upper unit of the boa vista formation found in the eastern part of the brazilian state of roraima and in neighboring guyana this unit has two geomorphologically sedimentologically distinct domains a sandy and a silty realm located to the east and southeast upwind of the silty realm the sandy realm includes very highly eroded inactive parabolic and linear dunes and sand sheets an extensive field of parabolic dune remnants was detected in an orbital radar image and in low altitude air photos of the interfluve of the cauame and uraricoeira rivers near the town of boa vista along the downwind periphery of the sandy realm is a thin mantle of silty sediments forming a flat plain and penetrating into intermontane valleys the eroded fossil dunes oriented 50 60 e and the downwind grain size selection suggest a late pleistocene dry period with trade wind direction the same as that of the modern dry season a more recent dry period probably holocene is suggested by the presence of many well preserved circular depressions in the upper unit of the boa vista formation interpreted as circular pans or blowout hollows