palynofacies and stable c and n isotopes of holocene sediments from lake macuco linhares espirito santo southeastern brazil depositional settings and palaeoenvironmental evolution particulate organic matter in sediments constitutes a valuable proxy for paleoenvironmental reconstructions a 192 cm lacustrine sediment core 7521 cal yr b p from the northern coast of the state of espirito santo southeastern brazil was radiocarbon dated and then analyzed for palynofacies stable isotopes c and n and grain size the values for total organic carbon toc 1 91 to 20 89 total nitrogen tn 0 08 to 1 1 13c 27 10 to 29 01 15n 1 92 to 5 09 and c n 12 87 to 50 11 suggest predominance of c3 plants and changing mixtures of terrestrial and aquatic organic matter in the sediments palynofacies and cluster analyses were used to identify four palynofacies associations that reflect different phases in the lake macuco evolution palynofacies 1 192 to 106 cm 7521 cal yr b p to 7054 cal yr b p interpolated age is characterized mainly by amorphous organic matter aom non opaque phytoclasts nops mangrove pollen grains rhizophora sp l 1753 and avicennia sp l 1753 and foraminiferal test linings palynofacies 2 106 to 65 cm 7054 cal yr b p interpolated age to 4847 cal yr b p is dominated by nops and aom with an increase in the values of mangrove pollen grains relative to palynofacies 1 both palynofacies reflect marine and fluvial influences in a depositional environment related to the development of an estuarine system during the mid holocene transgressive stage palynofacies 3 65 to 27 cm 4847 cal yr b p to 3800 cal yr b p interpolated age comprises high abundances of fluvial elements nop and opaque phytoclasts op and low values of aom the depositional environment is interpreted as being an estuary during the marine regression with the displacement of the estuarine basin and mangroves towards the sea palynofacies 4 27 to 0 cm 3800 cal yr b p is characterized by the predominance of aom and nop increase in the values of freshwater algae and no evidence of marine influence this last phase reflects the beginning of the establishment of lake macuco and floodplain of the barra seca river under conditions similar to the present day

palaeoreconstruction of the amazon river freshwater and sediment discharge using sediments recovered at site 942 on the amazon fan a continuous reconstruction of palaeoclimate has been generated for the last 12 000 c 14 yr bp from odp site 942 5 degrees 45 n 49 degrees 6 w at a water depth of 3346 m drilled to the west of the amazon fan records from the site suggest that during the younger dryas the amazon basin was extremely dry and amazon river discharge was low this increased aridity is hypothesised to be due predominantly to reduced precipitation in addition there is evidence for a discharge event at the end of the younger dryas coeval with termination 1b that produced an estimated amazon river outflow equivalent to present day values and an increase in sediments derived from the andes the timing of this event coincident with the warming of the andean ice sheet suggests that it was at least partly driven by meltwater produced by the retreat of andean glaciers but also required an increase in regional rainfall resulting from changes in climate site 942 also demonstrates that the sediment input to the western part of the amazon fan from the river ceased between 9900 and 9500 c 14 yr bp at a time when sea level was 40 50 m below the present level if this value is truly indicative of the sea levels at which the sediment supply to the fan switched off then it is far greater than the 30 m below current sea level suggested previously copyright c 2000 john wiley sons ltd

variability of el nino southern oscillation activity at millennial timescales during the holocene epoch the variability of el nino southern oscillation enso during the holocene epoch in particular on millennial timescales is poorly understood palaeoclimate studies have documented enso variability for selected intervals in the holocene but most records are either too short or insufficiently resolved to investigate variability on millennial scales1 2 3 here we present a record of sedimentation in laguna pallcacocha southern ecuador which is strongly influenced by enso variability and covers the past 12 000 years continuously we find that changes on a timescale of 2 8 years which we attribute to warm enso events become more frequent over the holocene until about 1 200 years ago and then decline towards the present periods of relatively high and low enso activity alternating at a timescale of about 2 000 years are superimposed on this long term trend we attribute the long term trend to orbitally induced changes in insolation and suggest internal enso dynamics as a possible cause of the millennial variability however the millennial oscillation will need to be confirmed in other enso proxy records

the amazon river plume during amasseds spatial characteristics and salinity variability the amazon river discharge forms a plume of low salinity water that extends offshore and northwestward over the north brazilian shelf observations acquired as part of a multidisciplinary amazon shelf sediment study amasseds are used to characterize the spatial structure and temporal variability of the amazon plume four shipboard conductivity temperature depth ctd surveys spanning the shelf from 1 s to 5 n during rising march 1990 maximum may 1990 falling august 1989 and minimum november 1991 discharge show the amazon plume is typically 3 to 10 m thick and 80 to over 200 km wide northwest of the river mouth the plume is often characterized by a wedge of low salinity water adjacent to the coast and a separate tongue of low salinity water extending offshore over the middle to outer shelf a bottom front separating the low salinity plume water from oceanic water is consistently located between the 10 and 20 m isobaths a moored array deployed about 300 km northwest of the river mouth from february to june 1990 included inner and midshelf moorings in 18 and 65 m of water on which temperature conductivity measurements were made the moored observations reveal salinity variations within the amazon plume of over 10 psu on timescales of days to weeks this variability includes intermittent events in which plume water pools up in the vicinity of the river mouth and the plume width can exceed 200 km these accumulation events are apparently due to wind events with a southeastward component which impede or block the normally northwestward freshwater transport the resulting bulges in the plume are then released northwestward when the wind reverses volume budgets indicate the amazon plume entrains roughly twice the river discharge between the river mouth at the equator and 3 n estimates of gradient richardson numbers from the moorings suggest entrainment due to the strong semidiurnal tidal currents occurs where the plume intersects the bottom and over the outer portion of the plume where salinities approach oceanic values

late glacial stage and holocene tropical ice core records from huascaran peru two ice cores from the col of huascaran in the north central andes of peru contain a paleoclimatic history extending well into the wisconsinan wurm glacial stage and include evidence of the younger dryas cool phase glacial stage conditions at high elevations in the tropics appear to have been as much as 8 to 12 c cooler than today the atmosphere contained about 200 times as much dust and the amazon basin forest cover may have been much less extensive differences in both the oxygen isotope ratio 18o 8 per mil and the deuterium excess 4 5 per mil from the late glacial stage to the holocene are comparable with polar ice core records these data imply that the tropical atlantic was possibly 5 to 6 c cooler during the late glacial stage that the climate was warmest from 8400 to 5200 years before present and that it cooled gradually culminating with the little ice age 200 to 500 years before present a strong warming has dominated the last two centuries

a vegetation and fire history of lake titicaca since the last glacial maximum fine resolution fossil pollen and charcoal analyses reconstruct a vegetation and fire history in the area surrounding lake titicaca 3810 m peru bolivia since ca 27 500 cal yr bp hereafter bp time control was based on 26 accelerator mass spectrometer ams radiocarbon dates seventeen ams dates and 155 pollen and charcoal samples between ca 17 500 bp and ca 3100 bp allow a centennial scale reconstruction of deglacial and early to mid holocene events local and regional fire signals were based on the separation of two charcoal size fractions 180 m and 179 65 m charcoal abundance correlated closely with the proportion of woody taxa present in the pollen spectra little or no pollen was detected in the sedimentary record prior to ca 21 000 bp very cold climatic conditions prevailed with temperatures suggested to be at least 5 8 c cooler than present increases in pollen concentration suggest initial warming at ca 21 000 bp with a more significant transition toward deglaciation ca 17 700 bp between 17 700 bp and 13 700 bp puna brava is progressively replaced by puna and sub puna elements the most significant changes between the pleistocene and the holocene floras were largely complete by 13 700 bp providing an effective onset of near modern conditions markedly earlier than in other andean records fire first occurs in the catchment at ca 17 700 bp and becomes progressively more important as fuel loads increase no evidence is found of a rapid cooling and warming coincident with the younger dryas chron a dry event between ca 9000 bp and 3100 bp with a peak between 6000 and 4000 bp is inferred from changes in the composition of aquatics and the marsh community as pollen of cyperaceae is replaced by poaceae apiaceae plantago and the shrub polylepis human disturbance of the landscape is evident in the pollen spectra after ca 3100 bp with the appearance of weed species

diastasis cracks mechanically generated synaeresis like cracks in upper cambrian shallow water oolite and ribbon carbonates upper cambrian limestones and dolostones of western newfoundland canada display conspicuous synsedimentary mud cracks cracks occur in carbonate mudstone interbedded with ooid and peloid grainstone unwashed oolite and ribbon rock lithofacies the traditional interpretation is that these are desiccation cracks the weight of evidence supports an alternative e explanation cracks resulted from the differential mechanical behaviour under stress of stiff mud interlayered with loose ooid peloid sand the processes envisaged to cause such diastasis cracks may be applicabl e to a wide variety of both carbonate and terrigenous clastic deposits composed of interlayered sediments of contrasting material properties and may be a viable alternative to synaeresis

mangrove history during middle and late holocene in pacific south eastern mexico a paleoecological analysis of a 6 m long sedimentary sequence from the la encrucijada biosphere reserve rble from southern pacific mexico provides detailed information about past vegetation chang

morphology of the itapeva to tramandai transgressive dunefield barrier system and mid to late holocene sea level change the surface morphology of the transgressive dunefield barrier extending from itapeva to tramandai along the northern littoral of rio grande do sul is examined and an attempt is presented to link morphological changes across the barrier to holocene sea level changes the 4 5 km wide holocene barrier displays two typical morphologies an inner part dominated by large scale continuous alongshore overlapping dunefield phases comprising sand sheets dunes deflation plains and precipitation ridges and an outer part dominated by discontinuous medium to small scale triangular to lobate transgressive dunefield phases cut by both active and relict palaeo creeks or washouts holocene sea level in the region rose to c 1 to 3 m above present reaching a maximum around 5100 years bp and then progressively fell to the present level we argue that the effect on barrier development was to suppress the development of a drainage system during the rising and maximum stages and encourage the development of an organized drainage system in the form of regularly spaced washouts during the falling period and that this change in sea level from rising to falling therefore produced the large scale differences in barrier morphology copyright 2006 john wiley sons ltd

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

the impact of quaternary sea level changes on the evolution of the amazonian lowland coring and acoustic surveying 3 5 khz in both rivers and lakes in central amazonia provide additional insights into the late quaternary hydrological and sedimentological development of the amazon river and its tributaries erosion and accumulation phases were found to be linked to quaternary sea level changes the low sea level phase during the last glacial maximum caused deep incision of the amazon river and erosion in major tributaries such as the rio negro 1500 km upstream from the amazon mouth a 3 5 khz profiling suggests a lowering of the water level by at least 30 m at manaus during that phase the slope of the amazon valley must have increased resulting in an increase in bed load transport capacity the subsequent sea level rise caused a backwater effect far upstream with silting up of the amazon valley and the tributary inflows former river systems changed into ria lakes the floodplains of the amazon river the varzea were formed approximately 5000 years ago when the sea level approached its present day level copyright 2009 john wiley sons ltd

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

palaeodrainage on marajo island northern brazil in relation to holocene relative sea level dynamics a large area in northeastern marajo island northern brazil has been characterized geomorphologically applying information acquired from landsat imagery this study was combined with detailed sedimentologic analysis of continuous cores which provided a record of depositional settings developed in this area through the holocene the results revealed well preserved meandering to anastomosed drainage networks of wide palaeochannels that were superimposed by a narrower palaeochannel system in both cases the sedimentary record consists of sands heterolithic deposits and muds locally rich in plant debris the strata are organized into fining upward successions that reach approximately 18 m thick in the wide channels and 4 m thick in the narrow channels sedimentary features suggestive of a coastal location for the wider palaeochannels and reworking of sediments by tidal currents include the prevalence of well to moderately sorted rounded to sub rounded fine to medium grained sands displaying foreset

syneresis cracks subaqueous shrinkage in argillaceous sediments caused by earthquake induced dewatering abstract syneresis cracks often confused with subaerial desiccation phenomena are traditionally ascribed to subaqueous shrinkage whereby salinity changes caused deflocculation of clay this and other previously proposed mechanisms fail to account for their occurrence in low energy typically non evaporitic facies stratigraphically sporadic distribution intrastratal formation under shallow burial depths variation in morphology degree of contraction generation of sedimentary dikelets as crack fills and deformation of dikelets and enclosing layers instead it is suggested that ground motion from strong synsedimentary earthquakes caused argillaceous sediments to dewater interbedded sands and silts to be almost simultaneously liquefied and injected into the resulting fissures and then these dikelets to be distorted comparative rarity of syneresis cracks in phanerozoic versus precambrian marine strata is considered to be primarily an evolutionary consequence of theological changes caused by increased organic binding of clay flocs in the water column greater input of organic matter into the sediment and the diversification of sediment dwelling bacteria and meiofauna in phanerozoic deposits

subaqueous shrinkage cracks in the devonian of scotland reinterpreted abundant lenticular straight to curving cracks on bedding planes are commonly interpreted as subaqueously formed shrinkage cracks syneresis cracks the two most influential examples being those in the green river formation picard 1966 and the devonian orcadian basin of scotland donovan and foster 1972 cracked surfaces in the orcadian basin deposits show all gradations between lenticular cracks and well developed polygons of subaerial origin some lenticular cracks are inferred to have formed during subaerial desiccation many of the simplest lenticular cracks also have the shapes of gypsum crystals and originated as pseudomorphs the gypsum crystals were often important in providing nuclei for later desiccation cracks more complex crack patterns of incipient to fully formed desiccation polygons have arisen from desiccation of surfaces containing gypsum crystals or pseudomorphs resulting from gypsum crystal dissolution cracks of all types are preserved by sand infills introduced by wind transport across dried up lake floors reassessment of the literature reporting subaqueous cracks in rocks shows that many of these cracks are likely to have originated in a similar way partly as gypsum crystals and partly by limited subaerial desiccation the remaining examples have either already been reinterpreted in other ways or are insufficiently documented to rule out a subaerial origin there is no clear evidence for subaqueously formed cracks being preserved in rocks

chronostratigraphy and radiocarbon age inversion in the holocene regressive barrier of parana southern brazil abstract twenty two 14 c datings were performed at the central sector of the parana coast to define holocene regressive barrier evolution the barrier pleistocene substratum was ascribed an age between 40 400 and 30 000 yr bp but it can also represent the penultimate sea level highstand during marine isotope stage 5e the holocene barrier samples provided ages between 8542 8279 and 2987 2751 cal yr bp and showed at least six age inversions that were related to age differences between in situ or low distance transported shells or trunk fragments and high distance transported vegetal debris wood fragments and organic matter samples the regressive holocene barrier age was 4402 4135 cal yr bp near the base and 2987 2751 cal yr bp near the top most of the vegetal remains were transported by ebb tidal currents from the estuaries to the inner shelf below wave base level during the mid holocene highstand they were transported onshore by storm waves and littoral currents during the sea level lowering after the sea level maximum and were deposited mainly as middle shoreface swaley cross stratification facies

millennial to secular time scale impacts of climate and sea level changes on mangroves from the doce river delta southeastern brazil this work integrates geomorphological sedimentological and palynological data with radiocarbon dating as well as 13c 15n and c n from sedimentary organic matter to provide a model of mangrove dynamics during the evolution of a wave dominated delta in southeastern brazil mangrove dynamics are analyzed within the context of millennial and secular climatic and sea level changes tidal flats positioned at highest limit of the intertidal zone along the edge of a lagoon sheltered by beach ridges were occupied by wetlands represented by mangrove and herbaceous vegetation during the mid holocene high sea level after considering the relative sea level fall and relatively higher fluvial sediment discharge during the last 6350 years progradation took place along this shoreline resulting in extensive beach ridge deposits that overlie transgressive muds this process led to loss of mangrove area similar dynamics were repeated at 3043 cal yr bp although in a relatively more distal i e seaward posi

ice core palaeoclimate records in tropical south america since the last glacial maximum ice core records spanning the last 25 000 yr from the tropical andes of south america are reviewed these records from quelccaya huascaran and sajama present a high temporal resolution picture of both the late glacial stage lgs and the holocene climatic and environmental conditions in the south american andes late glacial stage conditions at high elevations appear to have been cooler than today although the magnitude of the inferred cooling differs with the particular proxy used e g snowline depression pollen ice cores insoluble dust and anion concentrations in the ice cores reveal that lgs hydrological conditions in the tropics 9 s were much drier than today whereas in the subtropics 18 s lgs conditions were much wetter this probably reflects the migration of the tropical hadley cell in response to a different meridional temperature gradient low nitrate concentrations in the lgs ice from both huascaran and sajama suggest that the amazon basin forest cover may have been much less extensive discussed is the conundrum surrounding the use of d 18 o as a palaeothermometer in the tropics where temperatures exhibit little seasonal variation yet the ice core records suggest that d 18 o records temperature variations on decadal to millennial time scales finally evidence is presented for a strong twentieth century warming copyright 2000 john wiley sons ltd

relation between carbon isotopes of plants and soils on marajo island a large tropical island implications for interpretation of modern and past vegetation dynamics in the amazon region abstract we assess the relation between the contrasting vegetation types of rainforest open savanna and wooded savanna coexisting in close contact on marajo island at the mouth of the amazon river floristic and carbon isotopic characterizations of modern plants were combined with organic matter carbon isotope and grain size records of soil to characterize vegetation evolution at six locations on southeastern and northeastern marajo island and its relations to climate changes since the late pleistocene c3 plants contribute 100 of the biomass in the rainforest on post barreiras sediments site 1 no significant vegetation changes are evident in these places since at least 7860 cal yr b p rainforests on paleochannels sites 4 and 6 are protected from flooding by slightly elevated sandy levee and have flora very similar to site 1 these forests were formed since the early mid holocene after channel abandonment c3 grasses are predominant in open savanna areas sites 4 5 and 6 with less representation in wooded savannas sites 2 and 3 however c4 grasses despite having fewer species constitute significant biomass in the wooded 60 and open savanna vegetation areas especially during the dry season the reconstructions of past vegetation together with the distributions of modern vegetation allow prediction that climate changes to drier conditions can significantly influence the future marajo island landscape likely enabling expansion of c4 plants in the flooding zone and of trees in the rainforests

southward migration of the austral limit of mangroves in south america abstract temperature influences the global distribution of mangroves and global warming may be causing a poleward mangrove expansion sedimentary features pollen and isotopes data from six sediment cores as well as 14c datings indicated a marine transgression during the holocene and it contributed to the expansion of tidal flats occupied by saltmarshes environmental conditions suitable for mangroves development occurred on the study site during the holocene but according to 210pb and 14c dating the establishment of mangroves mainly represented by laguncularia trees only began between 1957 and 1986 ad on the studied tidal flats spatial temporal analysis based on satellite and drone images revealed a mangrove expansion of 10 ha in the study area between 2003 96 1 ha and 2019 106 1 ha nowadays in the study area saltmarshes mainly characterized by spartina and acrostichum are sharing tidal flats with mangroves represented by laguncularia 5 m tall and avicennia 11 m tall probably the absence of mangroves during the holocene followed by their establishment and expansion during the anthropocene in the subtropical zone is associated with a migration of the austral mangrove limit into the temperate zone caused by the gradual increase in winter temperatures this process may be related to a poleward mangrove migration since the late holocene caused by a natural holocene global warming however the industrial era warming must have intensified the mangrove expansion into temperate zones

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

sand injectites at the base of the coconino sandstone grand canyon arizona usa abstract in the grand canyon large tabular and wedge shaped sand filled cracks commonly occur at the base of the coconino sandstone penetrating downward into the coarse siltstones of the hermit formation all previous workers have casually identified the vertical sand filled cracks as desiccation cracks until now they have never been studied cracks and their associated features were found and examined at thirty locations and it was found that they have characteristics difficult to explain using desiccation mud cracks or large playa cracks as a model instead it was found the cracks have features commonly found in clastic dikes and sand injectites some lateral sand bodies associated with the cracks have clastic sill like characteristics liquefaction and injection of the basal coconino into the hermit is indicated by 1 macroscopic and microscopic banding flow structures within the cracks 2 bedded sandstone clast breccias in structureless sandstone lenses at the base of the coconino 3 lateral sand bodies which are connected to the vertical cracks 4 a zoned depth distribution of cracks about the bright angel fault zone 5 insufficient clay mineralogy and particle size for the hermit to crack by desiccation 6 preferred orientation of the cracks roughly perpendicular to the bright angel fault zone and several other features caution should be exercised when interpreting sand filled cracks as desiccation features i e mud cracks even if the interpretation fits well with accepted paleoenvironmental models

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

last glacial maximum in an andean cloud forest environment eastern cordillera bolivia climatically sensitive tropical ecosystems provide important information that may help us to fill the gaps in our knowledge concerning the evolution of rainforests during periods of full glaciation small changes in precipitation in the amazon basin have immediate consequences for the survival of the andean cloud forest because its dominant source of moisture today is the atlantic ocean a 40 k y lacustrine record from the eastern cordillera in bolivia in an endemic species rich and ecologically threatened region shows a dry last glacial maximum indicating a drastic decrease of the amazonian moisture source to explain this aridity we infer steep temperature gradients between the pole and equator in both hemispheres that would have reduced considerably the size and displacement of the intertropical convergence zone and the austral summer precipitation this major change in water supply induced a dramatic reduction in species diversity and suggests that the andean cloud forest did not provide refugia for tropical lowland taxa during full glacial times

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

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

environmental changes recorded in the holocene sedimentary infill of a tropical estuary abstract early to late holocene sedimentary infill of the potengi jundiai estuary brazil was studied in a 31 5 m long sediment core the borehole was drilled in a mangrove area within the central part of the flood tide delta the sedimentary record encompassing the last 10 kyr was studied for sediment texture and for the n alkanes composition of organic matter om sediments rich in om and bivalve shells accumulated during the period of sea level rise mostly in an intertidal environment of mangrove forest mudflat and subtidal estuarine flood delta the characterization of n alkanes c10 c33 and lignin methoxyphenols by means of direct analytical pyrolysis py gc ms technique allowed determining the sources of the extant om downcore fluctuations were observed in n alkanes and in several n alkane based indices such as long chain carbon preference index cpi c24 c33 long to short chain ratio l s and average chain length acl as well as in the relative proportion of lignin derived compounds the n alkanes distribution points to changing sources of om input to the estuarine sediments over most of the holocene period major inputs of terrestrial vegetation alternated with periods of mixed input from aquatic macrophytes as the rate of sea level rise changed over time the n alkanes distribution and the lignin markers were useful for reconstructing paleoenvironmental conditions particularly during the early holocene when the lower potengi river valley now an estuarine area could accommodate terrestrial forest vegetation before switching to halophytic mangroves as sea levels rose

variations of the amazonian rainforest environment a sedimentological record covering 30 000 years abstract two cores css 2 and css 10 were collected from two lakes situated in the carajas area 6 35 s 49 30 w that show different stages of silting up the results of geochemical and petrographic organic analyses backed up by 14 c radiometric dating lead to a reconstruction of the variations in the hydrological regime of these lakes over the last 30 000 years evidence is revealed for a period of drying up characterized by a sedimentary hiatus between 22 000 and 13 000 14 c years bp followed by a period of refilling between 13 000 and 11 000 14 c years bp 15 400 12 900 cal years bp the latter interval is primarily characterized by a mainly clastic sedimentary input rich in quartz kaolinite and iron hydroxides leading on to the authigenic formation of siderite this type of sedimentation implies the erosion of the small catchment basin during periods of transition between arid and humid climatic regimes from 8000 to 4000 14 c years bp 8900 4500 cal years bp as well as from 2700 to 1500 14 c years bp 2780 1360 cal years bp special hydrological conditions prevailed they are characterized in the sedimentary record by the presence of micro laminae containing wood charcoal interlayered with sponge spicule material this interlamination reflects the rapid alternation of dry and humid periods a comparison with the results obtained by pollen analysis shows that sedimentological indicators once filtered for strictly local effects can provide a full record of the regional scale variations in the environment

effects of sea level rise and climatic changes on mangroves from southwestern littoral of puerto rico during the middle and late holocene abstract we evaluated the response to sea level rise and climatic changes of the northeastern caribbean by establishing a palaeoenvironmental reconstruction of the flamenco lagoon located in the coastal side of the guanica dry forest a mab unesco reserve in the semi arid southwestern coast of puerto rico 17 57 10 31 n 66 50 39 30 w the reconstruction was based on pollen records sediments analysis 13 c 15 n c n data and ams 14 c dating of one sediment core the geology of the area is rocky calcareous karst with rocky outcrops and canyons with intermittent streams that move water during periods of high run off conditions the present lagoon is surrounded by rocky outcrops where dry forest vegetation is present on the terrestrial sides and sand dunes on the coastal side we propose that the studied lagoon developed after the stabilization of sea level rise in the region following three main stages 1 before 5400 cal yr bp sediments were accumulated according to small channels dynamics and trees shrubs and herbs were present the sedimentary organic matter was mainly sourced from terrestrial c 3 and c 4 plants 2 between 5400 and 4400 cal yr bp tidal flats were established and mangroves mainly represented by rhizophora trees migrated landward with sea level rise and established in the area and the sedimentary organic matter was sourced from terrestrial c 3 plants 3 around 4400 cal yr bp sea level was near the stabilization the endorheic minibasin was closed when a coastal sand dune was established as a result of drier climatic conditions mangroves were eliminated from the system as a result of the closure of the area by sand dunes and increased salinity in water due to high evaporative demand of the climate the pollen dominance of herbaceous and shrubby vegetation prevails until present throughout the lagoon development sedimentary organic matter was sourced from c 3 and c 4 plants as well as phytoplankton organic matter the closure of the lagoon and the drier conditions prevalent in the region during the late holocene which lasts until the present day and increased the contribution of salinity tolerant phytoplankton that characterizes the current conditions of the flamenco lagoon

vegetation dynamics during the late pleistocene in the barreirinhas region maranhao state northeastern brazil based on carbon isotopes in soil organic matter abstract the study place is in the barreirinhas region maranhao state northeastern brazil a vegetation transect of 78 km was studied among four vegetation types restinga coastal vegetation cerrado woody savanna cerradao dense woody savanna and forest as well as three forested sites around lagoa do caco located approximately 10 km of the transect soil profiles in this transect were sampled for 13c analysis as well as buried charcoal fragments were used for 14c dating the data interpretation indicated that approximately between 15 000 and 9000 14c yr b p arboreal vegetation prevailed in the whole transect probably due to the presence of a humid climate approximately between 9000 and 4000 3000 14c yr b p there was the expansion of the savanna probably related to the presence of drier climate from 4000 3000 14c yr b p to the present the results indicated an increase in the arboreal density in the area due to the return to a more humid and probably similar climate to the present the presence of buried charcoal fragments in several soil depths suggested the occurrence of palaeofires during the holocene the vegetation dynamic inferred in this study for northeastern brazil is in agreement with the results obtained in areas of amazon region based on pollen analysis of lake sediments and carbon isotope analysis of soil organic matter som implying than similar climatic conditions have affected these areas during the late pleistocene until the present

relative sea level and climatic changes in the amazon littoral during the last 500 years abstract an integrated approach focused on sedimentology geochemistry palynology c and n isotopes and radiocarbon dating of a sediment core from an herbaceous plain not flooded by tides of the amapa littoral near the amazon river mouth allowed identification of two phases with marine and terrestrial influences mangroves occurred over tidal mud flats with marine influence between 5610 5470 and 470 310 cal yr bp the absence of mangrove vegetation since 470 310 cal yr was followed by the transition of brackish water organic matter to terrestrial c3 plants also the geochemical data indicate a decrease in sea water influence during this last time interval likely the displacement of mangrove forest to lower surfaces was caused by a relative sea level fall that may be associated with drier conditions with less rainfall during the second part of the last millennium as suggested by this work slight relative sea level fluctuations caused by regional or global climatic change may affect significantly the current mangrove area

changing lake dynamics and sequence stratigraphy of synrift lacustrine strata in a half graben an example from the triassic ischigualasto villa union basin argentina well exposed triassic rift strata from the ischigualasto villa union basin nw argentina include a 80 to ca 515 m thick lacustrine dominated package that can be correlated across a half graben using key stratigraphic surfaces sequence boundaries lacustrine flooding surfaces and forced regressive surfaces the characteristics of the synrift lacustrine fill in different parts of the half graben have been examined and the mechanisms controlling sedimentation inferred a variety of sedimentary environments are recognized including volcaniclastic floodplain mildly saline lake and playa lake offshore lacustrine delta front to fluvial dominated and wave dominated deltas distributary and fluvial channel and interdistributary bay the succession can be divided into four stratigraphic sequences ss1 to ss4 the oldest of which ss1 contains volcaniclastic fluvial and saline lake deposits it is thickest close to the western border fault zone reflecting more rapid subsidence here accommodation exceeded sediment and water input during ss1 the second and third sequences ss2 and ss3 mark the onset of widespread lacustrine sedimentation reflecting a balance between accommodation creation and water and sediment fluxes sequences ss2 and ss3 are represented by offshore meromictic lacustrine and deltaic deposits the latter mostly sourced from the flexural and southern axial margins of the half graben the presence of stacked parasequences bound by lacustrine flooding surfaces is related to climatically induced lake level fluctuations superimposed on variable rates of subsidence on the controlling rift border fault zone the youngest sequence ss4 is represented by the deposits of littoral lacustrine and shallow shelf deltas distinguished by a change in lithofacies palaeocurrents and sandstone composition suggesting a switch in sediment supply to the footwall margin to the nw the change in the sediment source is related to reduced footwall uplift the possible presence of a relay ramp and or supply from a captured antecedent drainage network during ss4 the rate of creation of accommodation was exceeded by the sediment and water discharge the stratigraphic evolution of lacustrine strata in the half graben was mainly controlled by tectonic processes including subsidence rate and the growth and evolution of the border fault zone but changing climate inducing changes in water balance and lake level and autocyclic processes delta lobe switching were also important

the history of south american tropical precipitation for the past 25 000 years long sediment cores recovered from the deep portions of lake titicaca are used to reconstruct the precipitation history of tropical south america for the past 25 000 years lake titicaca was a deep fresh and continuously overflowing lake during the last glacial stage from before 25 000 to 15 000 calibrated years before the present cal yr b p signifying that during the last glacial maximum lgm the altiplano of bolivia and peru and much of the amazon basin were wetter than today the lgm in this part of the andes is dated at 21 000 cal yr b p approximately coincident with the global lgm maximum aridity and lowest lake level occurred in the early and middle holocene 8000 to 5500 cal yr b p during a time of low summer insolation today rising levels of lake titicaca and wet conditions in amazonia are correlated with anomalously cold sea surface temperatures in the northern equatorial atlantic likewise during the deglacial and holocene periods there were several millennial scale wet phases on the altiplano and in amazonia that coincided with anomalously cold periods in the equatorial and high latitude north atlantic such as the younger dryas

cambrian stratigraphy and depositional history of the northern indian himalaya spiti valley north central india recent work on himalayan tectonics indicates that prior to the cenozoic collision of india and asia an enigmatic cambrian ordovician event may have strongly infl uenced the regional geology of the himalaya stratigraphic and sedimentological analyses of well preserved cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the himalaya the parahio formation defi ned herein of the parahio valley spiti region in the tethyan himalaya of india is the best biostratigraphically resolved section of cambrian strata in the entire himalaya this formation consists of 1350 m of dominantly siliciclastic deltaic deposits the formation ranges from uppermost lower cambrian lungwangmiaoan stage to middle middle cambrian hsuchuangian stage representing a time span of 5 10 m y it contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross bedded fl uvial facies many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits the cycles are interpreted to have resulted from delta lobe switching based on a lack of systematic stratigraphic changes in cycle or facies thicknesses this paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep sea fl ysch to shallow marine tidalites in addition our paleoenvironmental analysis and paleocurrent data suggest that the uppermost lower to middle cambrian deposits of the lesser and tethyan himalaya are parts of the same ancient northward prograding fl uvial deltaic depositional system of the paleo tethys margin of india an angular unconformity with overlying ordovician conglomeratic rocks has considerable local relief with meter scale scours and a valley fi ll 100 m thick the scours have northeast southwest orientations which parallel both the paleocurrents in the underlying parahio formation as well as published paleocurrent readings from the coarse red beds of the overlying ordovician strata the cambrian ordovician unconformity is of regional extent and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in spiti is 15 m y our sedimentological analysis and associated paleocurrent data from the parahio formation along with additional data from units both above and below the unconformity indicate that published models portraying foreland basin development at this time with southward directed thrusting are problematic an alternate possibility that uplift took place south of the tethyan himalaya is also problematic because no published stratigraphic or structural evidence exists for such an uplift to the south for either the greater or the lesser himalaya lithotectonic zones

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

upper permian lacustrine organic facies evolution southern junggar basin nw china abstract the jingjingzigou lucaogou and hongyanchi formations of the southern junggar basin together comprise over 1000 m of organic rich lacustrine facies they record an overall evolution from relatively shallow evaporative lakes to freshwater lakes associated with fluvial systems the jingjingzigou formation is characterized by low toc and hi and biomarker features such as abundant carotane consistent with a specialized saline or hypersaline biota biomarker distributions in jingjingzigou formation extracts most closely resemble oils from the giant karamay oilfield the lucaogou formation represents one of the richest and thickest lacustrine source rock intervals in the world yet it contradicts conventional lacustrine source rock models in at least two important aspects first deposition occurred at middle paleolatitudes 39 43 n rather than in the tropics second limited nutrient supply in a drainage basin dominated by intermediate volcanic rocks appears to have caused low to moderate primary productivities stable salinity stratification and low inorganic sedimentation rates in a deep lake nonetheless resulted in deposits with up to 20 toc and hi near 800 the overlying hongyanchi formation has 1 5 toc but low hi and was deposited in freshwater oxic to sub oxic lakes

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

the m2 tide on the amazon shelf as part of a multidisciplinary amazon shelf sediment study amasseds moored and shipboard current measurements made over the amazon shelf during 1990 1991 have been analyzed to determine the dominant semidiurnal tidal constituent the m2 these results have been combined with coastal sea level data from within the amazon and para rivers the adjacent shelf and with satellite derived tidal elevation data from off the shelf to provide a more complete description of the m2 tide in this complex river shelf system near the amazon river mouth the m2 tide propagates across the shelf and through the mouth as a damped progressive wave with its amplitude decreasing and phase increasing upriver over the adjacent shelf north of cabo norte the m2 tide approaches a damped standing wave with large amplitudes greater than 1 5 m near the coast due to near resonance within the coastal embayment formed by the cabo norte shoal to the south and cabo cassipore to the north the observed m2 tidal currents are nearly rectilinear and oriented primarily across the local isobaths comparisons between tidal observations in both the north channel and the cabo norte cabo cassipore embayment and a simple variable width channel tidal model indicate that 1 most of the m2 tidal energy dissipation occurs over the mid and inner shelf in water depths less than 20 m and 2 fluid muds found there cause a significant reduction of order 50 in the effective bottom friction felt by the m2 tide the approximate resonant period of the cabo norte cabo cassipore embayment is 11 9 hours and at resonance the average energy dissipation per forcing period is roughly 2 2 times the average mechanical energy in the embayment this damping rate is large enough that the tidal amplification is rather insensitive to forcing frequency so that the response of the embayment to forcing over the semidiurnal band should be essentially the same the vertical structure of the m2 tidal current is examined at one outer shelf site located in 65 m water depth the observed semimajor axis increases logarithmically with height above bottom within the lowest 1 2 m and reaches a maximum in excess of 0 5 m s at approximately 11 m above bottom the mean ellipticity is small less than 0 1 and positive indicating clockwise rotation of a nearly rectilinear current and the semimajor axis is oriented within 10 of the local cross isobath direction the m2 phase increases with height above bottom with flood at the bottom leading flood at the surface by about 1 hour a simple local homogeneous tidal model with time and space dependent eddy viscosity simulates the observed near bottom velocity reasonably well however the model suggests that stratification above the lowest few meters may significantly affect the tidal boundary layer structure at this site the m2 energy flux onto the amazon shelf and into the amazon and para rivers has been estimated using current and surface elevation data and the best fit variable width channel model results the net m2 energy flux into the mouths of the amazon and para rivers is 0 47 1010w and 0 19 1010w respectively a net m2 energy flux of about 3 3 1010w occurs onto the shelf between the north channel of the amazon river and cabo cassipore this stretch of the amazon shelf accounts for about 1 3 of the global dissipation of the m2 tide

microbial mat mediated structures in the ediacaran sonia sandstone rajasthan india and their implications for proterozoic sedimentation abstract a strong influence of microbial mats on the physics of neoproterozoic sedimentation is explored within a 60 m thick stratigraphic interval within the sonia sandstone jodhpur group western india this marine interval is bounded by two terrestrial units its base being marked by a transgressive lag and the top by an unconformity progradation from upper neritic above fair weather wave base to supralittoral settings was later terminated by a transgression deposits of lower shoreface upper shoreface transition thus overlie supralittoral aeolian sandstones the basal contact of the former deposits being marked by another transgressive lag a wide spectrum and abundant examples of microbial mat or mat derived structures supports unusual cohesiveness within granular sand deposited in a high energy marine environment the cohesiveness is manifested in abundant preservation of several delicate primary structures and also in their replication in overlying beds even after they were subjected to high energy currents a low rate of sedimentation and severely restricted sediment reworking resulted from prolific mat growth and consequent depletion in the sediment budget was manifested in bedform migration and evolution these factors could also have influenced the sequence building pattern in a distinctive way as is already reported from several meso and neoproterozoic successions ubiquitous mat growth thus had the potential to impart significant distinctions to proterozoic clastic sedimentary successions

subsidence structures associated with subaerial desiccation crack piping and their role in drainage evolution on a drained proglacial lake bed hagavatn iceland abstract a range of subsidence structures associated with desiccation crack piping are described from the drained proglacial lake bed of hagavatn iceland subsurface drainage along desiccation cracks leads to the formation of small scale subsidence structures these subsidence structures include linear fissures located along the limbs of desiccation cracks and crater like depressions that form at crack intersections the controls on subsurface piping are discussed and the role of these subsidence structures in determining the pattern of surface drainage on the former lake bed is identified preservation potential of these sedimentary structure is considered to be high given renewed lacustrine sedimentation the potential for similar structures to be found in the geological record is considered likely and consequently these structures may be of importance in the interpretation of crack networks in the geological record as being of subaerial as opposed to subaqueous origin

the holocene alluvial plain of the middle amazon river brazil abstract the fluvial belt of the amazon river is a complex of quaternary sedimentary units of different ages and formation conditions present position of the channel and the morphology and size of the alluvial plain are related to neo tectonic lineaments in the study area the main channel of the solimoes is nearly straight the river may be considered anastomosed albeit with few islands and anabranches the main holocene units are the impeded floodplain and the channel dominated floodplain the channel dominated floodplain is a complex mosaic of fluvial forms including younger channels active sandbars islands levees scroll dominated plains and abandoned belts the impeded floodplain is a widespread unit characterised by a very flat surface and round or irregularly shaped lakes the impeded drainage flood plain is only partially affected by the present floods and even those parts affected by floods receive little or no sediment from the main channel more important sediment contribution is by alluvial plain channels from the solimoes branches that generate deltas in the impeded floodplain lakes the impeded floodplain is interpreted to be an earlier stage in the evolution of the holocene alluvial belt the formation was marked by widespread vertical accretion and a channel morphogenetically less active than at present the later stage is associated with an active main channel and the paranas and generation of a scroll dominated plain this very large river system had a complex and delayed response to variations in the hydrological conditions induced by changes in the regional climate during the late holocene

negligible microbial matground influence on pre vegetation river functioning evidence from the ediacaran lower cambrian series rouge france supported by shell international exploration and production b v under research framework agreement pt38181

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

late quaternary vegetation and climate change in the amazon basin based on a 50 000 year pollen record from the amazon fan odp site 932 hemipelagic sediments from the amazon deep sea fan odp site 932 5 12 7 n 47 1 8 w and continental shelf provide a 50 000 yr long pollen record of amazon basin vegetation the age model for hole 932a is constrained by eight magnetic remanence intensity features one paleomagnetic excursion and three ams14c dates alchornea melastomataceae myrtaceae and moraceae urticaceae are dominant taxa in the pollen record between 40 200 and 19 800 cal yr b p andean taxa such aspodocarpusandhedyosmum increase in abundance between 19 800 and 11 000 cal yr b p and prior to 40 200 cal yr b p the holocene pollen assemblage derived from amazon river and continental shelf sediments is dominated by secondary growth taxa such ascecropia climatic factors influencing the development of glacial and interglacial tropical vegetation are considered by comparing marine with terrestrial records of vegetation change this comparison shows that the amazon basin forests were not extensively replaced by savanna vegetation during the glacial period contradicting the refugia hypothesis

estimation of the thickness of anthropogenic deposits in historical urban centres an interdisciplinary methodology applied to rome italy in historical urban centres the superimposition of excavation and deposition activities over time has resulted in an irregular spatial distribution of anthropogenic deposits which may reach consi

dynamics flushing and trapping in hinchinbrook channel a giant mangrove swamp australia hinchinbrook channel is a 44 km long tidal channel connected at its northern and southern ends to the continental shelf of the great barrier reef the channel drains 164 km 2 of mangrove swamps it also drains the herbert river the dynamics within the channel and the flushing of the fringing mangrove swamps were studied both in the field and using numerical models the dominant currents are tidal semi diurnal and are due to two nearly identical tidal waves entering the channel at its norhtern and southern openings and meeting in the middle the mangrove swamps contribute significantly to the tidal dynamics when the herbert river discharge is small net residual currents are negligible even in the presence of wind and the residence time of water is about 2 months flushing of the fringing mangrove swamps is largely controlled by the trapping effect in the mangrove swamps and is slow with an e fold residence time of about 54 days on the northern end the mangrove fringed channel is wide and shallow and a barotropic coastal boundary layer is formed due to shallow water effects and trapping in mangrove swamps this results in a long term 2 weeks trapping of water along the mangrove fringed coast in calm weather thus coastal waters are continuously exchanged and mixed with mangrove swamp waters but mixing between coastal and offshore waters is much slower

pleistocene precipitation balance in the amazon basin recorded in deep sea sediments abstract terrigenous sediments from ceara rise in the western tropical atlantic ocean record pleistocene amazon basin climate variability iron oxides and oxyhydroxides in this region originate mainly from chemically leached amazon lowland soils concentrations of goethite and hematite in the terrigenous fraction consistently peak during transitions from glacial to interglacial periods suggesting an increased proportion of erosive products derived from the amazon lowlands compared to the physically weathered highlands lowland amazon basin precipitation changes monitored by the percentage of goethite relative to total iron oxides lead ice age extremes with maximum aridity during ice growth and maximum precipitation during ice melt rapid climate changes over the amazon basin may reflect shifts in the position of the intertropical convergence zone forced by northern hemisphere insolation at precessional 1 23 000 yr 1 and obliquity 1 41 000 yr 1 frequencies variance in the orbital eccentricity bands 1 100 000 and 1 413 000 yr 1 may be explained by nonlinear amplification of insolation forcing at precessional frequencies the early response of amazon precipitation to insolation ahead of high latitude ice volume 18o at all orbital frequencies suggests that tropical aridity is part of the chain of events leading to ice ages rather than a response to glacier oscillations

proliferation of miss forming microbial mats after the late neoproterozoic glaciations evidence from the kimberley region nw australia abstract a total of 20 morphological types of microbially induced sedimentary structures miss are recognized from the late neoproterozoic tillite bearing successions of the kimberley northwestern australia which include alpha petees triradiate cracks multidirectional linear ridges millimeter ripples erosional remnants and pockets and so on of these triradiate cracks multidirectional linear ridges and millimeter ripples are reported for the first time from the neoproterozoic worldwide most kimberley miss are morphologically comparable with their ancient and present day counterparts whose biogenicity have been well demonstrated in the kimberley during the interglacial period miss have colonization preference to relatively coarse sandy substrates ranford formation and their abundance decreased towards the younger strata throssell shale in contrast miss abundance increased from the older strata yurabi formation towards the younger strata flat rock formation during the postglacial period however overall both miss abundance and bedding plane coverage percentage increased from the marinoan younger marinoan gaskiers interglacial successions to younger marinoan gaskiers postglacial successions indicating a proliferation of microbial mats after the younger marinoan gaskiers glaciation the increase of microbial mats could have ameliorated marine biogeochemical conditions and ecosystems and thus laid the foundation for the rise of ediacaran biota in australian basins