late pleistocene and holocene vegetation changes in northeastern brazil determined from carbon isotopes and charcoal records in soils abstract northeastern brazil represents a strategic area in terms of quaternary records of environmental changes in south america due to its distinct semi arid climate in near equatorial latitudes in this study carbon isotope and charcoal distribution records in soils are used to characterize vegetation dynamics forest fires and their relation to climate change since the late pleistocene in the states of ceara piaui and paraiba northeastern brazil at the ceara site the carbon isotope record showed an enrichment trend from 24 to 19 during the early mid holocene indicating an opening of vegetation and expansion of savanna vegetation c 4 plants during this period a trend toward more depleted 13 c values 32 in the late holocene indicates an expansion of forest vegetation c 3 plants a similar trend is observed at the piaui and paraiba sites where values of 24 are associated with open forest vegetation during the late pleistocene in the early mid holocene 13 c values of up to 18 0 suggest the expansion of c 4 plants based on the carbon isotope data it is postulated that from 18 000 cal yr b p to 11 800 cal yr b p 10 000 cal yr b p arboreal vegetation was dominant in northeastern brazil and is associated with humid climates the savanna expanded from 10 000 cal yr b p to 4500 3200 cal yr b p due to a less humid drier climatic phase also supported by the significant presence of fires charcoal fragments in the soil from approximately 3200 2000 cal yr b p to the present carbon isotope records suggest forest expansion and a more humid phase these results form part of a regional pattern since they are in agreement with paleovegetation records obtained in regions of maranhao northeastern brazil and in the amazon and rondonia states northern brazil

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

applications of organic geochemistry to paleolimnological reconstructions a summary of examples from the laurentian great lakes the organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions evaluate histories of climate change and assess impacts of humans on local ecosystems the elemental isotopic and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area and they serve as proxies of organic matter delivery and accumulation sedimentary records from the north american great lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions the records of these lakes date from retreat of the laurentide ice sheet around 12 ka include the mid holocene hypsithermal and show consequences of recent human changes low corg ntotal ratios indicate that most of the sediment organic matter in the great lakes is from algal production yet changes in biomarker molecule compositions also show that varying amounts of land plant organic matter have been delivered to the lakes elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13c values in the organic matter of sediments that were deposited in the 1960s and 1970s increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the great lakes accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the great lakes emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising yet some potentially important measurements remain underutilized

a first order global model of late cenozoic climatic change ii further analysis based on a simplification of co2 dynamics a model developed recently for the long term variations of global ice mass carbon dioxide and mean ocean temperature through the late cenozoic is simplified by hypothesizing a new equation for the co2 variations containing one less adjustable parameter but retaining the essential physical content of the previous equation including nonlinearity and the potential for instability by assuming plausible time constants for the glacial ice mass and global mean ocean temperature and setting the values of six adjustable parameters rate constants a solution for the last 5 my is obtained displaying many of the features observed over this period including the transition to the near 100 ky major ice age oscillations of the late pleistocene in obtaining this solution it is also assumed that variations in tectonic forcing lead to a reduction of the equilibrium co2 concentration perhaps due to increased weathering of rapidly uplifted mountain ranges over this period as a consequence of this co2 reduction the model dynamical system can bifurcate to a free oscillatory ice age regime that is under the pacemaker influence of earthorbital milankovitch forcing expanded discussions are given of the surface temperature variations accompanying the evolution of ice co2 and ocean temperature and of the bifurcation properties of the model from both mathematical and physical viewpoints

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

bifurcation structure and noise assisted transitions in the pleistocene glacial cycles 1 the glacial cycles are attributed to the climatic response of the orbital changes in the irradiance to the earth these changes in the forcing are too small to explain the observed climate variations as simple linear responses nonlinear amplifications of the orbital forcing are necessary to account for the glacial cycles here an empirical model of the nonlinear response is presented from the model it is possible to assess the role of stochastic noise in comparison to the deterministic orbital forcing of the ice ages the model is based on the bifurcation structure derived from the climate history it indicates the dynamical origin of the mid pleistocene transition from the 41 ka world to the 100 ka world the dominant forcing in the latter is still the 41 ka obliquity cycle but the bifurcation structure of the climate system is changed the model suggests that transitions between glacial and interglacial climate are assisted by internal stochastic noise in the period prior to the last five glacial cycles while the last five cycles are deterministic responses to the orbital forcing

a conceptual model for glacial cycles and the middle pleistocene transition milankovitch s astronomical theory of glacial cycles attributing ice age climate oscillations to orbital changes in northern hemisphere insolation is challenged by the paleoclimatic record the climatic response to the variations in insolation is far from trivial in general the glacial cycles are highly asymmetric in time with slow cooling from the interglacials to the glacials inceptions and very rapid warming from the glacials to the interglacials terminations we shall refer to this fast slow dynamics as the saw tooth shape of the paleoclimatic record this is non linearly related to the time symmetric variations in the orbital forcing however the most pronounced challenge to the milankovitch theory is the middle pleistocene transition mpt occurring about one million years ago during that event the prevailing 41 kyr glacial cycles corresponding to the almost harmonic obliquity cycle were replaced by longer saw tooth shaped cycles with a time scale around 100 kyr the mpt must have been driven by internal changes in climate response since it does not correspond to any apparent changes in the orbital forcing in order to identify possible mechanisms causing the observed changes in glacial dynamics it is relevant to study simplified models with the capability of generating temporal behavior similar to the observed records we present a simple oscillator type model approach with two variables a temperature anomaly and a climatic memory term the generalization of the ice albedo feedback is included in terms of an effective multiplicative coupling between this latter climatic memory term representing the internal degrees of freedom and the external drive the simple model reproduces the temporal asymmetry of the late pleistocene glacial cycles and suggests that the mpt can be explained as a regime shift aided by climatic noise from a period 1 frequency locking to the obliquity cycle to a period 2 3 frequency locking to the same obliquity cycle the change in dynamics has been suggested to be a result of a slow gradual decrease in atmospheric greenhouse gas concentration the critical dependence on initial conditions in the non autonomous glacial dynamics raises fundamental questions about climate predictability

is the astronomical forcing a reliable and unique pacemaker for climate a conceptual model study there is evidence that ice age cycles are paced by astronomical forcing suggesting some kind of synchronisation phenomenon here we identify the type of such synchronisation and explore systematically its uniqueness and robustness using a simple paleoclimate model akin to the van der pol relaxation oscillator and dynamical system theory as the insolation is quite a complex quasiperiodic signal involving different frequencies the traditional concepts used to define synchronisation to periodic forcing are no longer applicable instead we explore a different concept of generalised synchronisation in terms of coexisting synchronised solutions for the forced system their basins of attraction and instabilities we propose a clustering technique to compute the number of synchronised solutions each of which corresponds to a different paleoclimate history in this way we uncover multistable synchronisation reminiscent of phase or frequency locking to individual periodic components of astronomical forcing at low forcing strength and monostable or unique synchronisation at stronger forcing in the multistable regime different initial conditions may lead to different paleoclimate histories to study their robustness we analyse lyapunov exponents that quantify the rate of convergence towards each synchronised solution local stability and basins of attraction that indicate critical levels of external perturbations global stability we find that even though synchronised solutions are stable on a long term there exist short episodes of desynchronisation where nearby climate trajectories diverge temporarily for about 50 kyr as the attracting trajectory can sometimes lie close to the boundary of its basin of attraction a small perturbation could quite easily make climate to jump between different histories reducing the predictability our study brings new insight into paleoclimate dynamics and reveals a possibility for the climate system to wander throughout different climatic histories related to preferential synchronisation regimes on obliquity precession or combinations of both all over the history of the pleistocene

lateglacial and holocene environmental changes in portuguese coastal lagoons 1 the sedimentological and geochemical records of the santo andre coastal area the sedimentological and geochemical study of a core corresponding to sedimentation in the santo andre lowland since 14 000 bp allowed the reconstruction of environmental changes of this area and discussion of the relative importance of local versus global forcing factors responsible for its evolution the pre holocene section of the core represents terrestrial sedimentation in a fluviatile environment contemporaneous of a low sea level and a distal shoreline sediment inputs were terrestrial and resulting from intensive weathering and high erosion rate in the adjacent watershed during the early holocene c 10 020 to 5380 bp the sea invaded this lowland and defined an open marine shallow environment corresponding to a wide gulf which has been disturbed by multiple terrestrial inputs that might have been induced by massive episodes of fluvial discharge the first symptoms of confinement were noticed close to 5400 bp and c 5380 bp a major environmental threshold occurred possibly reflecting the marke

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

crossover and peaks in the pleistocene climate spectrum understanding from simple ice age models the power spectrum provides a compact representation of the scale dependence of the variability in time series at multi millennial time scales the spectrum of the pleistocene climate is composed of a set of narrow band spectral modes attributed to the regularly varying changes in insolation from the astronomical change in earth s orbit and rotation superimposed on a continuous background generally attributed to stochastic variations quantitative analyses of paleoclimatic records indicate that the continuous part comprises a dominant part of the variance it exhibits a power law dependency typical of stochastic self similar processes but with a scale break at the frequency of glacial interglacial cycles here we discuss possible origins of this scale break the connection between the continuous background and the narrow bands and the apparently modest spectral power above the continuum at these scales we demonstrate that the observed scale break around 100 ka can have a variety of different origins and does not imply an internal time scale of correlation as implied by the simplest linear stochastic model

eolian depositional episodes controlled by late quaternary relative sea level changes on the imbituba laguna coast southern brazil the late quaternary coastal eolian deposits of the imbituba laguna region santa catarina state southern brazil comprise two eolian depositional sequences separated by an unconformity the older eolian sequence a is correlated to the relative sea level rise of the last interglacial period the eolian sequence b was initiated during the relative sea level rise in the post glacial period and extends up to active dunefields the correspondence between the two eolian depositional episodes represented by these sequences and the relative sea level highstand phases is confirmed by thermoluminescence and optically stimulated luminescence dating this correspondence is explained by a conceptual model that links the availability of coastal sediments for eolian transport with relative sea level changes the link between relative sea level changes and coastal eolian sedimentation can be used to integrate coastal eolian systems to the sequence stratigraphy model

a critical review of mid to late holocene sea level fluctuations on the eastern brazilian coastline abstract the eastern coast of brazil is 7000 km long and has been the subject of systematic paleo sea level investigations for more than 35 years more than 1000 samples have been radiocarbon dated and paleo sea level trends have been determined for 14 coastal sectors these trends have in common a mid holocene sea level maximum pmt above present sea level and a subsequent fall to the present time the time and elevation of the pmt the time when relative sea level rose above present mean sea level and the nature of late holocene sea level fall can however differ significantly discrepancies are observed not only between neighboring coastal sectors but also between different studies in a same coastal sector this paper discusses all the key radiocarbon dated samples used to establish regional trends in relative sea level on the eastern coast of brazil it is concluded that although many of the key sea level indicators are imprecise in time and space there is widespread evidence 30 of the data set for a progressive decline possibly with uneven rates of relative sea level since the end of the pmt

landscape evolution during the late quaternary at the doce river mouth espirito santo state southeastern brazil article the sedimentary deposits of the deltaicplain of the doce and barra seca rivers southeastern brazil were studied by faciesanalysis pollenrecords 13 c c n analysis and ams 14 c dating today thisdeltaicplain isdominated by beach ridges and sandy terraces occupied by arboreal and herbaceous vegetation between 47 500 and 29 400 cal yr b p a deltaic system was developed in response mainly to eustatic sea level fall although the studied stratigraphic succession is compatible with the trend of global sea level fall the earlier sea level sug gested by the topographic position of these deltaic deposits was above the one expected during the mis3 stage a tectonic uplift likely occurred during the late quaternary and raised these deposits the post glacial sea level rise caused a marine incursion with invasion of embayments and broad valleys and it favored the evolution of an estuary with wide tidal mud flats occupied by mangroves between 7400 and 5100 cal yr b p the high river sand supply and or the relative sea level fall in the late holocene led to seaward and downward translation of the shoreline during normal forced regression producing progradational deposits with shrinkage of mangrove stands and expansion of marshes colonized by herbaceous vegetation therefore the stratigraphic architecture and evolution of the doce river deltaic plain suggest that fluvial sediment supply and relative sea level fluctuations related to quaternary global climatic changes and local tectonism exerted major controls on sedimentation through the variation of accommodation space and base level changes

the middle pleistocene transition characteristics mechanisms and implications for long term changes in atmospheric pco2 abstract the emergence of low frequency high amplitude quasi periodic 100 kyr glacial variability during the middle pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system this middle pleistocene transition mpt began 1250 ka and was complete by 700 ka its onset was accompanied by decreases in sea surface temperatures ssts in the north atlantic and tropical ocean upwelling regions and by an increase in african and asian aridity and monsoonal intensity during the mpt long term average ice volume gradually increased by 50 m sea level equivalent whereas low frequency ice volume variability experienced a 100 kyr lull centered on 1000 ka followed by its reappearance 900 ka although as a broad band of power rather than a narrow persistent 100 kyr cycle additional changes at 900 ka indicate this to be an important time during the mpt beginning with an 80 kyr event of extreme sst cooling followed by the partial recovery and subsequent stabilization of long term north atlantic and tropical ocean ssts increasing southern ocean sst variability primarily associated with warmer interglacials the loss of permanent subpolar sea ice cover and the emergence of low frequency variability in pacific ssts and global deep ocean circulation since 900 ka ice sheets have been the only component of the climate system to exhibit consistent low frequency variability with the exception of a near universal organization of low frequency power associated with marine isotope stages 11 and 12 all other components show an inconsistent distribution of power in frequency time space suggesting a highly nonlinear system response to orbital and ice sheet forcing most hypotheses for the origin of the mpt invoke a response to a long term cooling possibly induced by decreasing atmospheric pco2 none of these hypotheses however accounts for the geological constraint that the earliest northern hemisphere ice sheets covered a similar or larger area than those that followed the mpt given that the mpt was associated with an increase in ice volume this constraint requires that post mpt ice sheets were substantially thicker than pre mpt ice sheets indicating a change in subglacial conditions that influence ice dynamics we review evidence in support of the hypothesis that such an increase in ice thickness occurred as crystalline precambrian shield bedrock became exposed by glacial erosion of a thick mantle of regolith this exposure of a high friction substrate caused thicker ice sheets with an attendant change in their response to the orbital forcing marine carbon isotope data indicate a rapid transfer of organic carbon to inorganic carbon in the ocean system during the mpt if this carbon came from terrigenous sources an increase in atmospheric pco2 would be likely which is inconsistent with evidence for widespread cooling apparently rapid carbon transfer from terrestrial sources is difficult to reconcile with gradual erosion of regolith a more likely source of organic carbon and nutrients which would mitigate pco2 rise is from shelf and upper slope marine sediments which were fully exposed for the first time in millions of years in response to thickening ice sheets and falling sealevels during the mpt modeling indicates that regolith erosion and resulting exposure of crystalline bedrock would cause an increase in long term silicate weathering rates in good agreement with marine sr and os isotopic records we use a carbon cycle model to show that a post mpt increase in silicate weathering rates would lower atmospheric pco2 by 7 12 ppm suggesting that the attendant cooling may have been an important feedback in causing the mpt

sensitivity experiments with a model of the ice age cycle the response to milankovitch forcing abstract we address the question of the extent to which a simple one dimensional time dependent climate model is able to explain the fluctuations of continental ice volume which have occurred during the last quarter of the pleistocene epoch the model includes explicit ice physics based on the glen flow law and explicit glacial isostatic adjustment based on a realistic viscoelastic field theory when the model is forced by the milankovitch variations of insulation it delivers ice volume fluctuations which are very similar to those implied by the specmap 18o record in particular the model appears capable of explaining the observed correlation between eccentricity and ice volume variations the dominance of the 105 year period in the power spectrum of the ice volume time series and the thickness and latitudinal extents of the continental ice sheets which existed at wurm wisconsin maximum 18 000 years ago

transgressions rethinking beringian glaciation abstract the purpose of this investigation is to encourage a fresh look at pleistocene beringia heretofore flooding of bering strait has been cited as the only barrier to migration with marine sea transgressions being a sea gate that closed off migration during glacial interstadials and interglaciations however the possibility exists that glacial advances were also barriers with marine ice transgressions being an ice gate that closed off migration during glacial stadials and glacial maxima this possibility proceeds from the marine ice transgression hypothesis mith which states that marine ice sheets form on the broad arctic continental shelf of northern hemisphere continents when sea ice thickens grounds and domes in shallow water and then transgresses landward as continental ice sheets and seaward as floating ice shelves hughes 1987 landward transgression is onto coastal lowlands during pleistocene glaciations a marine ice sheeet extending from spitsbergen to greenland may have transgressed the circumpolar continental landmass at its lowest and narrowest gap central beringia and calved into the pacific ocean four models of beringian glaciation are presented based on the distinction between marine glaciation and highland glaciation central beringia was glaciated only in highlands in the traditional model hopkins et al 1982 was also glaciated by a self sustaining ice shelf floating over the deep ocean basins of the bering sea in the model by grosswald and vozovik 1984 was glaciated by a marine ice sheet that covered highlands the continental shelf and supplied the ice shelf in a model for maximum pleistocene glaciation and was glaciated by a marine ice sheet in the chukchi sea that merged with highland glaciers transgressed the continental shelf of the western bering sea and calved into the southern bering sea along the edge of the continental shelf in a model for the last glaciation field tests are suggested to assess the viability of these four models the first model is already established for highland glaciation in alaska but less established in siberia the last model should be the easiest to evaluate for marine glaciation the last model limits human migration across the beringian land bridge to brief intervals between stadials and interstadials of the last glaciation cycle when both the ice gate and the sea gate were opened to human migration this model can influence the sea change now underway among quaternary scientists studying peopling of the americas based on the archaeological linguistic and ethnic diversity among native american populations

iron deficiency limits phytoplankton growth in antarctic waters enrichment experiments were performed in the ross sea to test the hypothesis that iron deficiency is responsible for the phytoplankton s failure to use up the luxuriant major nutrient supplies found in these and all other offshore antarctic ocean waters nitrate uptake rates in the controls without added trace elements ranged from 0 58 to 1 22 mol kg 1 d 1 the addition of 1 to 5 nmol of unchelated fe per liter resulted in rates that were 2 to 10 times higher 2 54 to 6 00 mol no3 kg 1 d 1 rates in bottles with 2 nmol mn added were identical to those in the controls 0 57 to 1 04 mol no3 kg 1 d 1 total decreases in no3 were balanced by increases in particulate organic n these results suggest that fe deficiency is the primary reason that the present day southern ocean biological pump is shut off in contrast iron was 50 times more abundant during the last glacial maximum greater fe availability may have stimulated the biological pump and contributed to the ice age drawdown of atmospheric c02 these results also imply that large scale southern ocean fe fertilization is feasible at least in terms of the total amounts of fe required i e 100 000 to 500 000 tons yr 1

late quaternary palaeoclimate in the savannas of south america the paper reviews late quaternary palynological evidence from eight sites in the savannas of northern south america the sites reviewed are lake valencia 10 n carajas 6 s aguas emendadas 15 s crominia 17 s salitre 19 s lagoa de serra negra 18 s lagoa santa 20 s and lagoa dos olhos 20 s four of the sites show evidence for a climate more humid than at present from ca 36 ka bp to ca 22 18 ka bp maximum dryness was reached at 14 ka to 10 5 ka bp at lake valencia there was an increase in moisture from 8 8 ka bp at lake valencia and from 7 ka bp in central brazil the presence of charcoal indicates human impact from ca 8 6 ka bp 1997 john wiley sons ltd

variation of coastal dynamics during the last 7000 years recorded in beach ridge plains associated with river mouths example from the central brazilian coast abstract the reconstruction of relative sea level changes during the last 7000 years has shown that the central brazilian coast was been subjected to submergence until approximately 5100 yr b p however the emergence after 5100 yr b p was interrupted by two important fluctuations with an amplitude of approximately 2 3 m and a duration of 200 300 yr this evolutionary history which is quite different from that of several other regions in the world played an essential role in the development of the central brazilian coastal plains whether or not they are associated with important river mouths the periods of submergence characterized by erosional phases introduced noticeable changes in the geometry of the coastal deposits the periods of emergence gave rise to sandy terraces covered by beach ridges whose orientation is determined by the longshore drift elsewhere on a low sandy coast the direction of longshore sand transport depends on the orientation of the swell in a coast subjected to various swell patterns the efficient swells are defined as those which determine the resulting longshore transport direction such swells are not necessarily the most prevalent for instance along the central brazilian coast the southern sector waves in spite of their infrequent occurrences are much more powerful than the northern sector waves and therefore generate a longshore sand transport from south to north when fossil beach ridges are present as is the case on the central brazilian coast their geometry reflects the past directions of longshore sand transport this makes it possible to determine the provenance of past efficient swells and to establish the past wind patterns a detailed study of the beach ridge geometry of some sections of the central brazilian coast showed a sequence of reversals of the longshore drift during the last 5100 yr with a duration ranging from 10 to 100 yr these reversals represent changes in the direction of effective waves which determine the longshore transportation of sediments and consequently indicate changes in the wind pattern

consequences of pacing the pleistocene 100 kyr ice ages by nonlinear phase locking to milankovitch forcing 1 the consequences of the hypothesis that milankovitch forcing affects the phase e g termination times of the 100 kyr glacial cycles via a mechanism known as nonlinear phase locking are examined phase locking provides a mechanism by which milankovitch forcing can act as the pacemaker of the glacial cycles nonlinear phase locking can determine the timing of the major deglaciations nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear a consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record nonlinear phase locking can also result in the observed quantization of the glacial period into multiples of the obliquity or precession periods

studies on holocene mangrove ecosystem dynamics of the braganca peninsula in north eastern para brazil abstract three sediment cores from the braganca peninsula located in the coastal region in the north eastern portion of para state have been studied by pollen analysis to reconstruct holocene environmental changes and dynamics of the mangrove ecosystem the cores were taken from an avicennia forest bosque de avicennia bda a salt marsh area campo salgado cs and a rhizophora dominated area furo do chato pollen traps were installed in five different areas of the peninsula to study modern pollen deposition nine accelerator mass spectrometry radiocarbon dates provide time control and show that sediment deposits accumulated relatively undisturbed mangrove vegetation started to develop at different times at the three sites at 5120 14c yr bp at the cs site at 2170 14c yr bp at the bda site and at 1440 14c yr bp at the fdc site since mid holocene times the mangroves covered even the most elevated area on the peninsula which is today a salt marsh suggesting somewhat higher relative sea levels the pollen concentration in relatively undisturbed deposits seems to be an indicator for the frequency of inundation the tidal inundation frequency decreased probably related to lower sea levels during the late holocene around 1770 14c yr bp at bda around 910 14c yr bp at fdc and around 750 14c yr bp at cs the change from a mangrove ecosystem to a salt marsh on the higher elevation around 420 14c yr bp is probably natural and not due to an anthropogenic impact modern pollen rain from different mangrove types show different ratios between rhizophora and avicennia pollen which can be used to reconstruct past composition of the mangrove in spite of bioturbation and especially tidal inundation which change the local pollen deposition within the mangrove zone past mangrove dynamics can be reconstructed the pollen record for bda indicates a mixed rhizophora avicennia mangrove vegetation between 2170 and 1770 14c yr bp later rhizophora trees became more frequent and since ca 200 14c yr bp avicennia dominated in the forest

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

glacial variability over the last two million years an extended depth derived agemodel continuous obliquity pacing and the pleistocene progression abstract an agemodel not relying upon orbital assumptions is estimated over the last 2 ma using depth in marine sediment cores as a proxy for time agemodel uncertainty averages 10 ka in the early pleistocene 2 1 ma and 7 ka in the late pleistocene 1 ma to the present twelve benthic and five planktic 18 o records are pinned to the agemodel and averaged together to provide a record of glacial variability major deglaciation features are identified over the last 2 ma and a remarkable 33 out of 36 occur when earth s obliquity is anomalously large during the early pleistocene deglaciations occur nearly every obliquity cycle giving a 40 ka timescale while late pleistocene deglaciations more often skip one or two obliquity beats corresponding to 80 or 120 ka glacial cycles which on average give the 100 ka variability this continuous obliquity pacing indicates that the glacial theory can be simplified an explanation for the 100 ka glacial cycles only requires a change in the likelihood of skipping an obliquity cycle rather than new sources of long period variability furthermore changes in glacial variability are not marked by any single transition so much as they exhibit a steady progression over the entire pleistocene the mean variance skewness and timescale associated with the glacial cycles all exhibit an approximately linear trend over the last 2 ma a simple model having an obliquity modulated threshold and only three adjustable parameters is shown to reproduce the trends timing and spectral evolution associated with the pleistocene glacial variability

late quaternary vegetation dynamics in the southern amazon basin inferred from carbon isotopes in soil organic matter abstract carbon isotopes of soil organic matter som were used to evaluate and establish the chronology of the vegetation dynamics of an ecosystem presently composed of savannas surrounded by forests the study was carried out on a 200 km transect along highway br 319 on the border of amazonas and rondonia states in southern amazon brazil large ranges in 13 c values were observed in som collected from profiles in the savanna 27 to 14 and forest regions 26 to 19 reflecting changing distribution of 13 c depleted c 3 forest and 13 c enriched c 4 savanna vegetation in response to climate change these results indicate that from about 17 000 to 9000 14 c yr b p the study area was covered by forest vegetation between approximately 9000 and 3000 14 c yr b p savanna vegetation expanded at the expense of the forest although the expansion of savanna did not occur with the same intensity along the study transect this process was very clearly registered by 13 c enrichment in the som since 3000 14 c yr b p the carbon isotope data suggest that forested regions have expanded this study adds to the mounting evidence that extensive forested areas existed in the amazon during the last glaciation and that savanna vegetation expanded in response to warm and dry conditions during the early to middle holocene

holocene palaeoenvironmental history of the amazonian mangrove belt abstract wetland dynamic in the northern brazilian amazon region during the holocene was reviewed using palynological carbon and nitrogen isotopes records and c n ratio previously published the integration of 72 radiocarbon dates recorded in 34 sediment cores sampled along the marine and fluvial littoral and mainly influenced by the amazon river reveals that marine influence and mangrove vegetation were wider than today on the mouth of amazon river between 8990 8690 and 2300 2230 cal yr bp forming a continuous mangrove belt along the northern brazilian amazon littoral the establishment of this mangrove strip is a direct consequence of the marine incursion caused by post glacial sea level rise possibly associated with tectonic subsidence during the early and middle holocene in the late holocene in areas influenced by the amazon river discharge the mangroves were replaced by freshwater vegetation and the coast morphology evolved from an estuarine dominated into a rectilinear coast due to coastal progradation nevertheless the marine influenced littoral which is currently dominated by mangroves and salt marsh vegetation has persistently had brackish water vegetation over tidal mud flats throughout the entire holocene likely the fragmentation of this continuous mangrove line during the late holocene was caused by the increase of river freshwater discharge associated to the change from dry into wet climates in the late holocene this caused a significant decrease of tidal water salinity in areas near the mouth of amazon river these changes in the amazon discharge are probably associated with dry and wet periods in the northern amazon region during the holocene

a phase space model for pleistocene ice volume article i nfo we present a phase space model that simulates pleistocene ice volume changes based on earth s orbital parameters terminations in the model are triggered by a combination of ice volume and orbital forcing and agree well with age estimates for late pleistocene terminations the average phase at which model terminations begin is approximately 90 90 before the maxima in all three orbital cycles the large variability in phase is likely caused by interactions between the three cycles and ice volume unlike previous ice volume models this model produces an orbitally driven increase in 100 kyr power during the mid pleistocene transition without any change in model parameters this supports the hypothesis that pleistocene variations in the 100 kyr power of glacial cycles could be caused at least in part by changes in earth s orbital parameters such as amplitude modulation of the 100 kyr eccentricity cycle rather than changes within the climate system

holocene environmental changes from the rio curua record in the caxiuana region eastern amazon basin abstract holocene environments have been reconstructed by multiproxy studies of an 850 cm long core from rio curua dating to 8000 14 c yr b p the low energy river lies in the eastern amazon rain forest in the caxiuana national forest reserve 350 km west of belem in northern brazil sedimentological mineralogical and geochemical dates demonstrate that the deposits correspond to two different environments sediments of an active river before 8000 14 c yr b p and later a passive river system the pollen analytical results indicate four different local and regional holocene paleoenvironmental periods 1 a transition to a passive fluvial system and a well drained terra firme unflooded upland amazon rain forest with very limited development of inundated forests varzea and igapo 7990 7030 14 c yr b p 2 a sluggish river with a local mauritia palm swamp and similar regional vegetation as before 7030 5970 14 c yr b p 3 a passive river forming shallow lake conditions and with still abundant terra firme forest in the study region 5970 2470 14 c yr b p and 4 a blocked river with high water levels and marked increase of inundated forests during the last 2470 14 c yr b p increased charcoal during this last period suggests the first strong presence of humans in this region the atlantic sea level rise was probably the major factor in paleoenvironmental changes but high water stands might also be due to greater annual rainfall during the late holocene

evidence of dry and cold climatic conditions at glacial times in tropical southeastern brazil last glacial paleoenvironments have been reconstructed from a pollen and charcoal record analyzed in organic sediments and dated between ca 18 000 and 48 00014c yr b p the site is located near the village catas altas in the lower highland region of southeastern brazil the last glacial landscape was covered by extensive areas of subtropical grasslands and small areas of gallery forests along the rivers where tropical semideciduous forests and cerrado ecosystems exist today the subtropical gallery forests were composed ofaraucariaforest trees such asaraucaria angustifolia podocarpus drimys ilex andsymplocos paleofires were frequent the record indicates that subtropical grassland vegetation which today is found in patches on the highlands in southern brazil especially in the state of santa catarina expanded from southern brazil to southeastern brazil over a distance of more than 750 km from latitudes of about 28 s to at least 20 s the completely different last glacial environment in comparison to the present day environment reflects a dry and cold climate with strong frosts during the winter months temperatures of 5 7 c below those of the present are inferred for the last glaciation

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

origin of step like and lobate seafloor features along the continental shelf off rio de janeiro state santos basin brazil a combined analysis of seismic and morphological features identified in a set of high resolution seismic reflection and bathymetric data shows a systematic relationship between major modern seafloor morphological traces and the basinward migration of late pleistocene coastlines along the continental shelf of the santos basin rio de janeiro state se brazil observed fairly continuous and sinuous mid outer shelf escarpments are related to the sea level variations and shelf exposure during the last glacial cycle a bathymetric step at 110 m is an erosional remnant of offlapping detached forced regressive wedges that spread over 50 km in the shelf dip direction probably developed during periods of falling sea level between mis 3 and 2 a second major escarpment at 130 m was interpreted as the shoreline during the lgm at the time of most extensive subaerial exposure of the continental shelf however a distal escarpment at 150 m is expressed as a straight contour feature along the two main shelf edge embayments that characterize the shelf break this escarpment is coupled with a basal seaward inclined and highly eroded ramp and was interpreted as the erosional action of bottom currents during the last transgression due to the displacement of the southward flowing brazil current towards the present day outer shelf previously published articles have regarded the morphological features observed on the modern shelf as indicators of stillstands during the post lgm transgression we conclude that on the contrary most of these features are actually from earlier parts of the late pleistocene and were formed in a regressive scenario under oscillating and relative slow sea level fall

late holocene intensification of colds fronts in southern brazil as indicated by dune development and provenance changes in the sao francisco do sul coastal barrier abstract holocene coastal barriers from southern brazil present great geomorphological changes during their late stages of development in this study we investigate the holocene evolution of the sao francisco do sul sfs barrier through geomorphological heavy minerals and grain size analyses constrained by optically stimulated luminescence osl dating the sfs barrier stands out among the southern brazilian barriers due to its well preserved morphology with a parabolic dune belt in the seaward portion the progradation of the sfs barrier started at least 4914 478 years ago and had a pronounced morphodynamic shift around 1891 155 years ago this shift is characterized by episodic development of parabolic dunes migrating to nnw associated with sand coarsening and a marked variation in sediment provenance represented by the input of sands derived from local coastal watersheds southward from the sfs barrier this morphodynamic provenance shift resulted from the strengthening of sse winds and associated wave systems responsible for the northward alongshore drift implying intensification of cold fronts coupled with higher precipitation since 1891 155 years ago osl dating combined with grain size heavy minerals and geomorphological analyses allowed assessing the response of coastal barriers to the impacts of centennial to millenial climate events occurred during the late holocene reconstruction of the late holocene climate variability and associated impacts on coastal sedimentation is a key issue to evaluate the sensitivity of coastal barriers to future climate changes

a multi proxy evidence for the transition from estuarine mangroves to deltaic freshwater marshes southeastern brazil due to climatic and sea level changes during the late holocene abstract the present study investigates a paleo estuary at the doce river delta southeastern brazil through a multi proxy approach that links palynology diatoms sedimentology and geochemistry analyses i e total c total n 13 c and 15 n these analyses temporally synchronized with five radiocarbon ages revealed environmental changes from marine to continental over the last 7550 years the studied sedimentary succession recorded the upward transition from estuarine channel until 7550 cal yr bp to estuarine central basin 7550 to 5250 cal yr bp deposits the latter containing increased mangrove vegetation marine and marine brackish water diatoms the range of geochemical values 13 c 30 10 15 n 2 8 and c n 4 40 also indicate marine estuarine organic matter and c 3 terrestrial plants to that time interval a following period recorded two phases lake ria 5250 to 400 cal yr bp and fluvial channel 400 cal yr bp until modern age during this stage mangroves were replaced by trees shrubs and herbs grasses due to the disconnection with the marine realm as a result the corresponding sediments contain only organic matter sourced from freshwater and c 3 terrestrial plants 13 c 29 26 15 n 0 8 and c n 10 45 the equilibrium between fluvial sediment supply and relative sea level changes during the holocene controlled the morphologic and vegetation changes in the studied littoral the estuary became established during the early holocene as a resulted of a eustatic sea level rise when the fluvial sediment supply to the coast was relatively lower due to a dry period however during the late holocene the climatic force was more significant to the development of coastal morphology due to a wet period that caused an increase in sandy sediment supply to coastal system then the increase of fluvial discharge associated to a relative sea level fall caused a marine regression and shrinkage of mangroves during the late holocene the evaluation of mangrove dynamics according to climatic and sea level changes mainly during the late holocene is essential for the understanding of their survival ability under future scenarios with a probable accelerated sea level rise and intensification of extreme climatic events in southeastern brazil for the next century

mangrove vegetation changes on holocene terraces of the doce river southeastern brazil article i nfo high resolution sedimentological geochemical and pollen analysis on sediment core from the coastal plain of the doce river southeastern brazil revealed changes in the depositional system and vegetation caused by combined action of oscillations in relative sea level rsl and sedimentary supply during the holocene two main phases were discerned using sedimentary features 13 c 15 n total organic carbon toc total ni trogen n c n and cluster analysis of pollen data temporally synchronized with radiocarbon age dating the data indicates the presence of a lagoon system surrounded by a tidal plain colonized by mangroves and its sedimentary organic matter sourced from c4 plants between 8050 and 7115 cal yr bp however during the mid and late holocene the mangroves shrank and freshwater vegetation expanded c3 plants proba bly due to a marine regression during this phase the development of a lacustrine environment was followed by the colonization of herbs trees and shrubs the continuous sediment infilling into the lake allowed the ex pansion of a herbaceous plain as seen today this geomorphologic and vegetation evolution is in agreement with the mid holocene rsl maximum above present rsl and subsequent fall to the present time

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

holocenic proxies of sedimentary organic matter and the evolution of lake arari amazon region abstract four sediment cores were sampled from lake arari located on marajo island at the mouth of the amazon river the island s vegetation cover is composed mainly of amazon coastal forest herbaceous and varzea vegetation the integration of data on sedimentary structures pollen carbon and nitrogen isotope records c n ratios and radiocarbon ages allowed the identification of changes in vegetation and the sources of organic matter accumulated in the lake during the holocene the data indicate a relatively high flow energy marine water influence and the presence of mangroves during the lagoon phase between 8990 and 8690 cal yr b p and 2310 2230 cal yr b p between 2310 and 2230 cal yr b p and 1000 cal yr b p the flow energy decreased and the mangroves were replaced by herbaceous vegetation following the decline in marine influence likely due to the increase in freshwater river discharge during the last 1000 years lake arari was established in association with the expansion of herbaceous vegetation and the dominance of freshwater algae

related changes in biodiversity insolation and climate in the atlantic rainforest since the last interglacial abstract the long term colonia record is located in the atlantic rainforest domain in brazil 23 52 s 46 42 20 w 900 m a s l the 780 cm long core co3 provides a coverage of a complete interglacial glacial cycle for the first time in a neotropical rainforest information on the behavior of tropical climates compared to global changes in temperatures indicates specific climate responses in terms of precipitation at these latitudes winter extratropical circulation was very active during the last interglacial and most of the glacial floristic composition of the rainforest changed several times in each phase of expansion twice during the interglacial and three times during glacial episodes araucaria was well developed in the area of sao paulo until the beginning of the first dry phase of the glacial at ca 50 000 yr b p changes in insolation controlled the expansion of the rainforest and the tropical hydrological cycle as evidenced by a strong precession signal however precession had no impact on regional climatic features the two interglacials mis 5e and holocene showed completely different patterns attesting to the continuous evolution of the forest the biodiversity index shannon wiener index remained high during both the interglacial and glacial attesting to the permanence of small patches of rainforest refugia during drier phases the lowest shannon wiener indexes were recorded between 23 000 and 12 000 yr b p and 40 000 and 30 000 yr b p and characterize two marked phases of stress for the rainforest

on the mid pleistocene transition to 100 kyr glacial cycles and the asymmetry between glaciation and deglaciation times 1 a mechanism is proposed for the mid pleistocene transition from a dominant periodicity of 41 kyr to 100 kyr in glacial oscillations the same mechanism is shown to also explain the asymmetry between the long glaciation and short deglaciation phases of each cycle since that transition versus the symmetry of the 41 kyr oscillations prior to the transition these features arise naturally within the framework of the sea ice switch glacial cycle mechanism of gildor and tziperman 2000 as a result of the gradual cooling of the deep ocean during the pleistocene this cooling results in a change of the relation between atmospheric temperature and the rates of accumulation and ablation of continentals ice sheets it is this latter change that leads to the activation of the sea ice switch and therefore to the initiation of the 100 kyr oscillations the gradual glaciation and rapid deglaciations during these oscillations occur because the mean value of the ice sheet ablation is not far from the maximum rate of snow accumulation during warm periods this proximity of mean ablation and maximum accumulation rates is shown to be also a consequence of the mid pleistocene gradual cooling of the deep ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 1620 global change climate dynamics 3309 1640 global change remote sensing 4267 oceanography general paleoceanography keywords climate dynamics glacial cycles mid pleistocene transition 100 kyr 41 kyr

are the 41 kyr glacial oscillations a linear response to milankovitch forcing abstract the characteristics of glacial oscillations changed drastically 0 8 ma ago at the mid pleistocene transition during the past 0 8 ma the 100 kyr glacial interglacial oscillations were strongly asymmetric i e long glacial intervals of growth followed by rapid intervals of deglaciation the 40 kyr oscillations prior to the mid pleistocene transition were of a smaller amplitude and less asymmetrical looking the smaller amplitude apparently symmetric form and period that matches that of obliquity suggests that these oscillations were a linear response to milankovitch forcing while the 100 kyr oscillations are attributed either to some nonlinear self sustained variability due to a mechanism internal to the climate system itself or to nonlinear amplification of the insolation forcing the significant strengthening of the 100 kyr eccentricity power in the past 800 kyr is one of the intriguing questions of climate history here we show that glacial interglacial oscillations pre mid pleistocene transition are in fact significantly asymmetric this asymmetry may contradict a straight forward linear milankovitch explanation and we therefore suggest that the glacial oscillations before and after the transition may both be explained as self sustained variability although the possibility of nonlinear response to insolation forcing still exists the role of milankovitch forcing is in setting the phase of the oscillations e g time of terminations and their period rather in being the main driving force of the oscillations this is demonstrated using a simple model based on the sea ice switch mechanism of gildor and tziperman paleoceanography 15 2000 605

the deep sea sediment record and the pliocene pleistocene boundary abstract the history of 18 o in planktonic and benthonic foraminifera from deep sea sediment cores is useful both as a tool for stratigraphic correlation and as a means of deriving the history of global ice volume it is also valuable as an indication of the pattern of climatic variability the reliability of this indicator has been demonstrated over the last few years as high resolution records for such diverse areas as the chinese loess plateau the highland of central america and the woodland of europe has been obtained kukla et al 1990 hooghiemstra et al 1993 wijmstra and groenhart 1983 follieri et al 1988 reille and de beaulieu 1990 for this reason any decision to erect new formal boundaries in the plio pleistocene should be based on an understanding of the appropriate part of the isotope record several 18 o records are now available that cover the entire plio pleistocene with the same temporal resolution that was until recently available only for the last few glacial cycles in addition a complete and robust astronomically calibrated timescale has been generated for the entire pliocene as well as the pleistocene both in open ocean deep sea sediments and in the classic pliocene and lower pleistocene sections that are well exposed in sicily and calabria shackleton et al 1990 1995a hilgen 1991a b the ultimate resolution and reliability of biostratigraphic datums is being developed and evaluated as these high resolution stratigraphic records become available raffi et al 1993 backman and shackleton 1983

wetland dynamics of marajo island northern brazil during the last 1000 years abstract the wetland dynamics in eastern amazon region during the last thousand years were studied by pollen analysis and ams radiocarbon date three sediment cores were sampled from the marajo island which is located at the mouth of the amazon river marajo island is covered mainly by amazon coastal forest herbaceous and varzea vegetation one core was obtained from lake arari which is surrounded by herbaceous vegetation flooded by freshwater two cores were sampled from an herbaceous plain that lies 45 km southeast from the lake arari the pollen records indicate that the herbaceous vegetation began its development approximately 800 cal yr b p the pollen percentages between 750 and 500 cal yr b p indicate the expansion of mangroves the predominance of herbaceous and varzea pollen occurs between 500 and 200 cal yr b p and the varzea pollen percentage increased during the last 200 years the vegetation changes proposed by the pollen records from the lake arari probably represent the regional vegetation dynamics of the marajo island while the pollen profiles from herbaceous plain present a smaller spatial representativeness of the vegetation the alternation between fresh and brackish water vegetation in the wetlands may indicate changes in the physicochemical characteristics of the sediment during the time interval studied

the development of blowouts and foredunes in the ilha comprida barrier southeastern brazil the influence of late holocene climate changes on coastal sedimentation abstract middle to late holocene barriers are conspicuous landforms in southeastern and southern brazilian regions the barriers in the coastal zones of northern santa catarina parana and sao paulo states 27 19 24 00 s are formed mainly by beach ridge alignments and many barriers present foredune and blowout alignments in their seaward portion the development of these eolian landforms appears to record a regional shift in coastal dynamics and barrier building in this context the ilha comprida barrier stands out for its well developed and well preserved foredunes and blowouts based on the presence or not and type of eolian landforms the ilha comprida barrier can be divided seaward into inner middle and outer units the inner unit is formed entirely by beach ridges the middle unit comprises a narrow belt of blowouts up to 15 m high aligned alongshore blowout lobes pointing nnw are indicative of their generation by southern winds the outer unit is represented by low 1 m high active or stabilized foredunes and a small transgressive dunefield 1 km 2 twenty seven luminescence ages sar protocol obtained for the beach ridges foredunes and blowouts of these three units allow definition of a precise chronology of these landforms and calculation of rates of coastal progradation the inner unit presents ages greater than 1004 88 years the blowouts of the middle unit show ages from 575 47 to 172 18 years the ages of the outer unit are less than 108 10 years rates of coastal progradation for the inner and outer units are 0 71 0 82 m year and 0 86 2 23 m year respectively the main phase of blowout development correlates well with the little ice age lia climatic event these results indicate that southern winds in subtropical brazil became increasingly more intense and or frequent during the lia these conditions persist to the present and are responsible for the development of the eolian landforms in the outer unit thus barrier geomorphology can record global climatic events the sensitivity of barrier systems in subtropical brazil to late holocene climate changes was favored by the relative sea level stillstand during this time luminescence dating makes it possible to analyze barrier geomorphology during late holocene climate changes operating on timescales of a hundred to thousand years these results improve our knowledge of barrier building and will help in the evaluation of the impact of future climate changes on coastal settings

beach ridges definitions and significance abstract beach ridges frequent components of quaternary coastal plains and other coastal landforms have been cited as indicators of the positions of ancient seashores and associated sea levels numerous authors utilized the term beach ridge for active and relict usually wave built supratidal and or intertidal forms wind built ridges have been only occasionally included in the definition the term was applied also to submerged landward shifting eventually stranded bars a consistent redefinition of the term is highly desirable beach ridges should include all relict strandplain ridges whether dominated by wave swash built or by eolian lithosomes all active ridge like shore features regardless of dimensions morphology and origin are excluded because of the resistance of coarse clastic ridges to wave and wind erosion swash built gravel or coarse shell storm ridges may build several meters above the level of high tide swash built high berms even on pure sandy beaches exceed the highest tides during episodes of wind induced record water levels frequently but not always burying underlying low relief berm ridges of berm lithosomes sequences of relatively steep multiple foredunes are commonly named beach ridge plains the narrow subparallel relict foredunes that form these strandplains presently are designated as eolian beach ridges beach ridges thus are defined as relict semiparallel multiple wave and wind built landforms that originated in the inter and supratidal zones until separated from the shoreline by progradation sandy pebbly or shell enriched backshore berm ridges behind an active foreshore should not be considered beach ridges strandplain progradation is either continuous or with the inclusion of subtidal cat s eye ponds discontinuous contrary to claims transgressive cheniers do not represent true cheniers alone within their overall progradational context cheniers a special category of beach ridges bracketed by subtidal intertidal mudflats may be transgressive or regressive in character landward driven transgressive ridges should be designated beach ridges only after they are stabilized on intertidal flats when recognizable between clearly identifiable intertidal and overlying eolian intervals the horizontal interface between these lithosomes in beach ridges may help the reconstruction of ancient tide lake levels diagnostic sedimentary textures structures and fossils however often may be unavailable in the deposits along with various types of elevated terraces composed of raised marine deposits and certain coastal landforms of erosional origin that occur worldwide beach ridges of clearly proven wave built origin may also serve as indicators of ancient higher than present sea levels

the evolution of a tropical rainforest grassland mosaic in southeastern brazil since 28 000 14c yr bp based on carbon isotopes and pollen records the lack of paleoecological records from the montane atlantic rainforest of coastal brazil a hotspot of biological diversity has been a major obstacle to our understanding of the vegetational changes since the last glacial cycle we present carbon isotope and pollen records to assess the impact of the glaciation on the native vegetation of the serra do mar rainforest in sao paulo brazil from ca 28 000 to 22 000 14c yr bp a subtropical forest with conifer trees is indicative of cool and humid conditions in agreement carbon isotopic data on soil organic matter suggest the presence of c3 plants and perhaps c4 plants from 28 000 to 19 000 14c yr bp the significant increase in the sedimentation rate and algal spores from 19 450 to 19 000 14c yr bp indicates increasing humidity associated to an erosion process between 19 000 and 15 600 14c yr bp from 15 600 14c yr bp to present there is a substantial increase in arboreal elements and herbs indicating more humid and warmer climate from 19 000 to 1000 14c yr bp 13c values indicated the predominance of c3 plants these results are in agreement with studies in speleothems of caves which suggest humid conditions during the last glacial maximum

the antarctic ice sheet and the triggering of deglaciations a new physical mechanism involving the antarctic ice sheet extent is able to link climatic and co2 glacial interglacial changes it is furthermore able to explain many features of the glacial cycles like the 100 kyr oscillations or the peculiarities of stage 11 about 400 kyr bp indeed from recent results the glacial ocean bottom waters were possibly much more saline and consequently may have an unsuspected large density this glacial deep stratification could account for a significant part of the glacial interglacial co2 difference the formation of these waters around antarctica involves brine rejection over the continental shelves and is directly linked to changes in sea ice formation and antarctic ice sheet extent using this new scenario we formulate a conceptual model that reproduce the succession and the amplitude of glaciations over the last few million years this new mechanism furthermore provides the clue to understanding the changes in the main climatic frequency from 23 to 41 ka about 3 myr ago and from 41 to 100 ka about 1 myr ago the antarctic ice sheet influence on bottom water formation could provide the bmissing pieceq to complete the astronomical theory of quaternary climates d 2004 elsevier b v all rights reserved

the middle pleistocene transition by frequency locking and slow ramping of internal period the increase in glacial cycle length from approximately 41 to on average 100 thousand years around 1 million years ago called the middle pleistocene transition mpt lacks a conclusive explanation we describe a dynamical mechanism which we call ramping with frequency locking rfl that explains the transition by an interaction between the internal period of a self sustained oscillator and forcing that contains periodic components this mechanism naturally explains the abrupt increase in cycle length from approximately 40 to 80 thousand years observed in proxy data unlike some previously proposed mechanisms for the mpt a rapid increase in durations can be produced by a rapid change in an external parameter but this assumes rather than explains the abruptness in contrast models relying on frequency locking can produce a rapid change in durations assuming only a slow change in an external parameter we propose a scheme for detecting rfl in complex computationally expensive models and motivate the search for climate variables that can gradually increase the internal period of the glacial cycles

early pleistocene glacial cycles and the integrated summer insolation forcing long term variations in northern hemisphere summer insolation are generally thought to control glaciation but the intensity of summer insolation is primarily controlled by 20 000 year cycles in the precession of the equinoxes whereas early pleistocene glacial cycles occur at 40 000 year intervals matching the period of changes in earth s obliquity the resolution of this 40 000 year problem is that glaciers are sensitive to insolation integrated over the duration of the summer the integrated summer insolation is primarily controlled by obliquity and not precession because by kepler s second law the duration of the summer is inversely proportional to earth s distance from the sun

late quaternary vegetation and coastal environmental changes at ilha do cardoso mangrove southeastern brazil abstract a 190 cm mangrove sediment core from the ilha do cardoso state park state of sao paulo southeastern brazil was analyzed for pollen diatoms as well as carbon and nitrogen isotopes the goal was to determine the dynamics of the coastal terrestrial aquatic ecosystems vegetation history and climate change in this region of the brazilian atlantic rainforest during the late pleistocene and holocene the values for total organic carbon toc from 3 up to 40 c n ratios from 10 up to 130 and 15n 0 to 8 are associated with well preserved aquatic and terrestrial organic matter and possibly influenced by nitrogen cycling e g denitrification that caused 15n enrichment between 40 000 cal yr b p and 23 000 cal yr b p depleted 13c values 28 0 are also observed and indicate the predominance of c3 plants during this time interval the pollen analysis reveals the presence of the genera ilex alchornea weinmannia myrsine symplocos drimys and podocarpus on a site currently occupied by mangrove vegetation these data suggest that in the past prevailed a colder and more humid climate than today with a low relative sea level from 23 000 cal yr b p to 2200 cal yr b p a sedimentary hiatus likely occurred related to an erosive event associated to the post glacial sea level rise since at least 2200 cal yr b p sediments are marked by relatively low c n ratios from 2 to 27 exhibit more enriched 13c from 26 0 to 24 0 and 15n up to 7 values and are characterized by the presence of marine diatoms this indicates the return of the marine coastal line to its current position and consequently the development of mangrove

the brazilian sea level curves a critical review with emphasis on the curves from the paranaguaand cananeia regions abstract relative sea level curves for the mid to late holocene have been proposed for eight different sectors along the brazilian coast this paper aims to review the data used to construct the curves proposed for two sectors the paranaguaand cananeia regions in southeast brazil more specifically we analyze the palaeo sea level indicators vermetid tubes shells wood fragments and shell midden deposits related to 1 the sea level maximum of the mid holocene and 2 the two secondary oscillations that would have occurred between 4100 3800 years b p and 3000 2700 years b p the sedimentary deposits of the coastal plain as well as vermetid datings suggest a mid holocene sea level maximum between 3 5 and 4 m the great majority of the indicators used to infer the secondary oscillations in previous studies were derived from shell middens besides being unreliable palaeo sea level indicators these data also showed inconsistencies that undermine the existence of such oscillations the plot of all the data derived from vermetid samples found along the brazilian coast also indicate a rather smoother sea level fall in the last 5100 years

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

the upper parana river brazil geomorphology sedimentology and paleoclimatology this study involves the geomorphological sedimentological and paleoclimatological aspects of the upper parana river at porto rico state of parana brazil the alluvial plain is composed of a braided system main channel with island and sandy bars and an anastomosing system involving secondary channels tributaries and a complex of swamps pools and natural levees river discharge in its upper course varies from 8400 to 13 000 m3 s minimum of 2550 m3 s and maximum of 33 740 m3 s solid discharge reaches 27 106 tons year for suspended sediments and 3 106 tons year for load sediments through echobathymetric survey bed forms were grouped in ripples megaripples dunes and sand waves a detailed study of the sedimentology heavy minerals and facies architecture was applied to the two major geomorphic provinces and their subenvironments channel province with sand bar and bed form deposits and overbank province with natural levee flood basin crevasse and island deposits palynological preliminary studies show that during the late pleistocene the area was dominated by grassland and savannas under drier climatic conditions since the beginning of the holocene there has been a generalized transition to a humid phase and the present broadleaf forest occupies the area some hypotheses can be formulated for the evolution of the area 1 the first order architectural element channel ch was generated during the pleistocene associated with climatic changes and tectonism the alluvial valley was filled up by alluvial colluvial deposits at the end of the pleistocene channel deposits present 40 ka bp 2 tectonism and climatic changes atlantic climatic optimum generated a new valley bottom 5 8 m below the former level and a wide meandering plain was built up 4000 4500 bp 3 a recent fluvial system created a new terrace 3 m above the normal water level

holocene sea level history on the rio grande do norte state coast brazil abstract the rio grande do norte state coast northeastern brazil lies in an intraplate region which displays elevated holocene shorelines abandoned tidal flats and other intertidal deposits a field and chronological investigation which used 48 radiocarbon dates on unaltered mollusk shells peats coral reefs and vermetids in living position and death assemblages encrusted on beachrock tidal flat and peat deposits was carried out along two littoral zones one trending east west and the other north south the sea level envelope curve for the region indicates at least one sea level oscillation after the maximum holocene highstand which occurred at 5000 cal yr bp this curve shows notable deviations from the mean sea level curve for the central brazilian coast in the early and mid holocene however in general this envelope curve fits a glacio isostatic prediction for the area a significant deviation from this prediction related to a small sea level oscillation occurred from 2100 to 1100 cal yr bp the comparison between the glacio isostatic prediction and the sea level observation enables us to conclude that local events may have disturbed the sea level record minor holocene sea level oscillations which were mainly associated with climate changes or tectonics and superimposed on a major smooth pattern were observed in south africa and australia but in northeastern brazil neotectonic controlled crustal movements or variation in marine and wind currents could provide the answer the data indicate that local events may have an important role in the history of coastal progradation and retreat