brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

bauxitisation of anorthosites from central brazil petrological studies using x ray diffraction xrd x ray fluorescence xrf optical microscopy scanning electron microscopy sem eds and electron microprobe analyzer wds showed the mineralogical micromorphological and geochemical transformations due to the bauxitisation of anorthosite from the barro alto stratiform mafic ultramafic complex central brazil the hydrolytic alteration of the anorthosite occurred in two different stages in accordance with the order of stability of the minerals to the weathering firstly the bytownite and secondly the ferromagnesian minerals the weathering solutions benefited from the existing network of fractures percolated the weakness zones of the minerals characterising the microsystem of contact in which cores of plagioclase and ferromagnesian minerals were formed in the first stage the plagioclases are altered directly to gibbsite and at an early stage during the change process the gibbsite crystals surround the primary ferromagnesian minerals that are totally or partially preserved the transformation is isalteritic and is responsible for formation of porous alteromorphs consisting of septa of coarse gibbsite and fine gibbsite as the weathering process advances the ferromagnesian minerals directly enter to goethite the boxworks of gibbsite and goethite characterise a primary plasmic microsystem

an ice core indicator of antarctic sea ice production 1 the sea ice surface not open water is the dominant source of sea salt to aerosol and ice cores in coastal antarctica here we show that it may also form the dominant source for central antarctica we can then explain higher concentrations in the winter and last glacial maximum lgm as being due to increased sea ice production this suggests that ice core sea salt can indicate at least the timing of changes in antarctic sea ice production the pattern of sea salt in ice cores is consistent with marine evidence about sea ice changes in the holocene and lgm sea salt shows no change at the initial co2 increase out of the last glacial making it unlikely this was primarily due to changing sea ice cover the sea salt record should not be treated as an indicator of meridional transport

the role of biological agents in the microstructural and mineralogical transformations in aluminium lateritic deposit in central brazil abstract petrological studies using x ray diffraction xrd and micromorphological analyses optical microscopy and sem were done to understand the role of biological activity in the evolution of barro alto bauxite the results indicated that this influence came through structural or microstructural and mineralogical transformations namely i the bioturbation caused by termites and ii the mechanical degradation and geochemical transformation promoted by roots in the bioturbation caused by termites were formed i a intergrain micro aggregate structure characterised by gibbsite crystals from isalteritic bauxite fragmentation on the bottom of the profile and ii a granular structure characterised by a termitic microaggregates with fragments of gibbsite immersed in a kaolinite gibbsite goethite boehmite micromass formed by bioturbation of the degradation clay with nodules of gibbsite whose origin is the geochemistry degradation of isalteritic bauxite the processes associated with geochemical and mechanical degradation caused by roots were responsible for the genesis of i a porphyric texture with bauxite fragments surrounded by nonaggregate material and ii fine monic structure where the gibbsite neoformation has the mould cavities left by old roots generating riziform features

glaciochemistry of polar ice cores a review human activities have already modified the chemical composition of the natural atmosphere even in very remote regions of the world the study of chemical parameters stored in solid precipitation and accumulated on polar ice sheets over the last several hundred thousand years provides a unique tool for obtaining information on the composition of the preindustrial atmosphere and its natural variability over the past this paper deals with the chemistry of polar ice focused on the soluble mineral na nh4 k ca mg h f cl no3 so4 and h2o2 and organic methanesulfonate ch3so3 formate hcoo acetate ch3coo and formaldehyde hcho species and their interpretation in terms of past atmospheric composition aerosols and water soluble gaseous species we discuss ice core dating the difficulties connected with trace measurements and the significance of the ionic composition of snow we examine temporal from the last decades back to the last climatic cycle and spatial including examples from coastal as well as central areas of greenland and antarctica variations in the ionic budget of the precipitation and evaluate ice core studies in terms of the chemical composition of our past atmosphere we review 1 how greenland and antarctic ice cores that span the last few centuries have provided information on the impact of human activities and 2 how the chemistry of deep ice cores provides information on various past natural phenomena such as climatic variations glacial interglacial changes el nino volcanic eruptions and large boreal forest fires

340 years of atmospheric circulation characteristics reconstructed from an eastern antarctic peninsula ice core 1 precipitation delivery mechanisms for dolleman island di located off the east coast of the antarctic peninsula are investigated using reanalysis and back trajectory data the southern annular mode sam and enso are both shown to influence precipitation delivery and event size precipitation delivery variability is compared against the interannual variation of chemical data from two di ice cores nitrate concentration in the cores is strongly linked with the ratio of easterly to westerly back trajectories arriving at di as described by a cross peninsula index cpi defined in this paper this cpi is used subsequently to reconstruct the atmospheric circulation characteristics for the 340 year ice core record the analysis highlights a period of increased easterlies during 1720 1780 and an increase in westerlies for 1950 1980 the latter concomitant with a positive sam trend and western peninsula warming the reconstruction also reveals periods when polynyas may have been present in the weddell sea

north atlantic oscillation dynamics recorded in greenland ice cores carefully selected ice core data from greenland can be used to reconstruct an annual proxy north atlantic oscillation nao index this index for the past 350 years indicates that the nao is an intermittent climate oscillation with temporally active coherent and passive incoherent phases no indication for a single persistent multiannual nao frequency is found in active phases most of the energy is located in the frequency band with periods less than about 15 years in addition variability on time scales of 80 to 90 years has been observed since the mid 19th century

the little ice age climate of new zealand reconstructed from southern alps cirque glaciers a synoptic type approach little ice age lia austral summer temperature anomalies were derived from palaeoequilibrium line altitudes at 22 cirque glacier sites across the southern alps of new zealand modern analog seasons with temperature anomalies akin to the lia reconstructions were selected and then applied in a sampling of high resolution gridded new zealand climate data and global reanalysis data to generate lia climate composites at local regional and hemispheric scales the composite anomaly patterns assist in improving our understanding of atmospheric circulation contributions to the lia climate state allow an interrogation of synoptic type frequency changes for the lia relative to present and provide a hemispheric context of the past conditions in new zealand an lia summer temperature anomaly of 0 56 c 0 29 c for the southern alps based on palaeo equilibrium lines compares well with local tree ring reconstructions of austral summer temperature reconstructed geopotential height at 1 000 hpa z1000 suggests enhanced southwesterly flow across new zealand occurred during the lia to generate the terrestrial temperature anomalies the mean atmospheric circulation pattern for summer resulted from a crucial reduction of the hse blocking synoptic type highs over and to the west of nz largely settled conditions and increases in both the t and sw trough synoptic types lows passing over nz enhanced southerly and southwesterly flow relative to normal associated land based temperature and precipitation anomalies suggest both colder and wetter than normal conditions were a pervasive component of the base climate state across new zealand during the lia as were colder than normal tasman sea surface temperatures proxy temperature and circulation evidence were used to corroborate the spatially heterogeneous southern hemisphere composite z1000 and sea surface temperature patterns generated in this study a comparison of the composites to climate mode archetypes suggests lia summer climate and atmospheric circulation over new zealand was driven by increased frequency of weak el nino modoki in the tropical pacific and negative southern annular mode activity

bipolar changes in atmospheric circulation during the little ice age annually dated ice cores from siple dome west antarctica and central greenland indicate that meridional atmospheric circulation intensity increased in the polar south pacific and north atlantic at the beginning 1400 a d of the most recent holocene rapid climate change event the little ice age lia as deduced from chemical concentrations at these core sites the lia was characterized by substantial meridional circulation strength variability and this variability persists today despite strong evidence for an end to lia cooling thus increased late 20th century storm variability may be in part a result of the continuation of these climatic fluctuations

decadal timescale links between antarctic peninsula ice core oxygen 18 deuterium and temperature the antarctic peninsula region has exprienced a long term warming trend over the twentieth century with the 1971 90 mean at faraday being 1 9 c warmer than the mean over 1903 41 based on expedition reports for the period prior to 1900 there is conflicting evidence from different data sources an initial interpretation of isotopic data from ice cores suggests that the nineteenth century was warmer than the twentieth century in contrast snow accumulation rate data for the nineteenth century from the same ice cores suggest lower temperatures here we investigate these facts by studying the links between atmospheric temperature over the antarctic peninsula circulation parameters and isotopic data over the period of instrumental records we show that the relationships between these variables are complex and highly spatially variable in particular the correlations between temperature and 18o and d are generally of the order r 0 5 or less on timescales of one to five years conflicts between evidenc

mineralogical micromorphological and geochemical evolution of the facies from the bauxite deposit of barro alto central brazil abstract the hydrolytic alteration of anorthosite from the barro alto stratiform mafic ultramafic complex central brazil caused the formation of an isalteritic bauxite petrological studies using x ray diffraction xrd x ray fluorescence xrf mass balance calculation and micromorphological description via optical microscopy were undertaken to understand the evolution of the bauxitic massif the results suggest the transformation of the isalteritic bauxite f1 into varied alteration facies and their filiations as follows the formation of semi compact massive bauxite f2 and compact massive bauxite f3 by the recrystallisation of gibbsite filing voids resulting in enriched absolute aluminium 9 and 25 respectively the formation of laminar bauxite f4 and fragmentary bauxite f5 which is associated with different types of fragmented massive duricrust such as those formed by mechanical root activity tectonics etc and geochemical dissolution of gibbsite kaolinite neoformation etc processes the formation of palaeopediment bauxite f6 the formation of degraded clay with gibbsitic nodules f7 which is associated with the resilicification process by the oscillation of the water table caused by changes in the shapes of slopes and the cycling of silica by vegetation isalteritic clay f8 facies directly derived from the anorthosite are also found the relationship between the facies reflects the polygenetic evolution of the bauxitic massif

the impact of a changing southern hemisphere annular mode on antarctic peninsula summer temperatures since the mid 1960s rapid regional summer warming has occurred on the east coast of the northern antarctic peninsula with near surface temperatures increasing by more than 2 degrees c this warming has contributed significantly to the collapse of the northern sections of the larsen ice shelf coincident with this warming the summer southern hemisphere annular mode sam has exhibited a marked trend suggested by modeling studies to be predominantly a response to anthropogenic forcing resulting in increased westerlies across the northern peninsula observations and reanalysis data are utilized to demonstrate that the changing sam has played a key role in driving this local summer warming it is proposed that the stronger summer westerly winds reduce the blocking effect of the antarctic peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern antarctic peninsula when this occurs a combination of a climatological temperature gradient across the barrier and the formation of a fohn wind on the lee side typically results in a summer near surface temperature sensitivity to the sam that is 3 times greater on the eastern side of the peninsula than on the west sam variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula similar to 62 degrees s both at the surface and throughout the troposphere this is in contrast to a station farther south similar to 65 degrees s where the sam exerts little influence

distribution and genesis of bauxite on the mambilla plateau se nigeria abstract only small low grade occurrences of bauxite are known in nigeria associated with ferricretes on the jos plateau widespread bauxite has been discovered on the mambilla plateau laterite is developed on gneisses and basalt while on trachytic parent rocks low silica bauxite occurs with a thickness of more than 5 m and covering an area of at least 1 km2 this bauxite consists of gibbsite and goethite in contrast to the ferricretes of the jos plateau weathering on the mambilla plateau led to the formation of stone line profiles ferricretes were developed on the upper jurassic gondwana surface under contrasting climate and are preserved as relicts in the southern part of the mambilla plateau where weathered precambrian basement occurs in a highly uplifted position dismantling of these ferricretes is attributed to a gradual change of paleoclimate towards more humid conditions after the extrusion of volcanic rocks in the northern part of the plateau presumably during lower to mid miocene these volcanics underwent intense chemical weathering bauxite and laterite formed depending on the type of parent rock weathering profiles are mostly truncated and overlain by reworked material allochthonous components occur both within and also below the stone line this is shown by mineralogical indicators such as ilmenite or quartz and by a shift in the zr ti ratio similar profiles are missing further north on the jos plateau where ferricretes formed instead this is attributed to regional differences in paleoclimatic conditions which were more humid at mambilla while jos was under the influence of dryer conditions the widespread formation of laterite on different parent rocks on the mambilla plateau could be related to a global mid miocene climatic optimum which led to a general intensification of chemical weathering processes

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

differing source water inputs moderated by evaporative enrichment determine the contrasting δ18ocellulose signals in maritime antarctic moss peat banks 1 oxygen isotope palaeoclimate records preserved in moss tissue cellulose are complicated by environmental influences on the relationships between source water inputs and evaporative conditions we carried out stable isotope analyses of precipitation collected from the maritime antarctic and cellulose extracted from co located chorisodontium aciphyllum dominated moss peat bank deposits accumulated since 1870 a d analyses of stable oxygen and hydrogen isotope composition of summer precipitation on signy island 60 7 s 45 6 w established a local meteoric water line lmwl similar to both the global mwl and other lmwls and almost identical to the hadam3 isotope enabled global circulation model output the oxygen isotopic composition of cellulose 18oc revealed little temporal variation between four moss peat banks on signy island since 1870 however 18oc followed two patterns with sites a and d consistently 3 enriched relative to 18oc values from sites b and c the growing moss surfaces at sites a and d are likely to have been hydrated by isotopically heavier summer precipitation whilst at sites b and c the moss banks are regularly saturated by the isotopically depleted snow melt streams laboratory experiments revealed that evaporative enrichment of c aciphyllum moss leaf water by 5 occurred rapidly following saturation ecologically equivalent to post rainfall or snow melt periods in addition to the recognized source water cellulose fractionation extent of 27 3 such a shift would account for the 32 difference measured between 18o of signy island precipitation and cellulose

a positive trend in western antarctic peninsula precipitation over the last 50 years reflecting regional and antarctic wide atmospheric circulation changes in situ observations of precipitation days days when snow or rain was reported in routine synoptic observations from faraday vernadsky station on the western side of the antarctic peninsula and fields from the 40 year european centre for medium range weather forecasts re analysis era 40 project are used to investigate precipitation and atmospheric circulation changes around the antarctic peninsula it is shown that the number of precipitation days is a good proxy for mean sea level pressure mslp over the amundsen bellingshausen sea the annual total of precipitation days at the station has been increasing at a statistically significant rate of 12 4 days decade 1 since the early 1950s with the greatest increase taking place during the summer and autumn this is the time of year when the southern annular mode sam has experienced its greatest shift to a positive phase with mslp values decreasing in the antarctic coastal zone the lower pressures in the circumpolar trough have resulted in greater ascent and increased precipitation at faraday vernadsky

ferricrete biochemical degradation on the rainforest savannas boundary of central african republic in southeastern central african republic the lateritic weathering mantles are capped by 2 to 5 m thick ferricretes which previously formed under a seasonal tropical climate the actual humid tropical climatic conditions result in the biophysical disaggregation of the ferricretes everywhere the forest develop that lead to a soil formation composed of ferricrete relicts and ferruginous nodules embedded in a soft bioturbated micro aggregated clay ferruginous matrix the potential effect of the biological activity e g termites on the soil and vegetation dynamics and therefore on the evolution of the previous consolidated ferricrete is discussed following its mechanical disaggregation by the forest tree roots the ferricrete underwent a chemical degradation under the combined effect of hydration and redox conditions which result from the biodegradation and oxidation of the organic matter the transformation of the ferricrete into a soft bioturbated micro aggregated clay ferruginous soil matrix implies the hematite dissolution and the kaolinite transformation into gibbsite which may characterize a late bauxitization secondary gibbsitization of kaolinite of previous ferricrete profiles linked to a late quaternary environmental climate and vegetation change however the geochemical and mineralogical patterns of the matrix not only reflect the geochemistry of the parental ferricrete but also depend on the physical transfers of quartz and heavy minerals from the lower horizons of the profile hence the use of geochemical indexes such as ti and or zr for mass balance calculations in the lateritic weathering profiles is precluded because these supposedly inert chemical elements are not simply accumulated by in situ chemical weathering process

a 340 year record of biogenic sulphur from the weddell sea area antarctica detailed records of methanesulphonic acid msa and non sea salt sulpha te nss so have been obtained from ice cores drilled on dolleman island on the east coast of the antarctic peninsula i0 35 2 s 60 55 5 w annual average concentrations of msa arc presented for the period i 652 1992 over this lime span the mean ann ual concen tration of msa is 0 69 eq 1 1 cr 0 33 n 340 the range is 0 13 2 35 eq 1 1 a nd the msa nss s04 2 ratio is 0 22 the high msa concentration reflects the proximity of the weddell sea believed to be a region of high marine phyt0plankton production t he overa ll mean nss so concentration is about 66 of the total sulphate d eposited in the snowfall low frequency variations of msa and oxygen isolope signals correla te closely indicat ing that they may be modulated by similar atmospheric processes positive correlations are observed between the oxygenisotope signature and both l 1sa r 0 4 1 and nss s04 2 r 0 50 signilicant at the 99 level a small negative correlation can be seen between both species and the annual duration or sea ice a t scoria bay laurie island in the south orkneys since 1902 msa r 0 23 and nss so r 0 29 significant at 95 confidence no significant link between high msa concentrations and el nino even ts is observed at this location

the contribution of termites to the microgranular structure of soils on the uasin gishu plateau kenya abstract the structure of many clay soils in tropical africa is composed of microaggregates that range in size from 0 03 to 1 mm these structural units are often very stable and known as pseudo sand or pseudo silt their presence is detected from the loamy feel of the soil this paper tests the hypothesis that termites contribute to a stable microgranular i e finely aggregated soil structure a termite mound was selected on pedologically unaffected clayey weathering material derived from phonolite the fate of microaggregates produced by termites has been followed through the various stages of mound building field evidence was supplemented with microscopic analysis development of the mound begins when lumps of soil are detached from the subsoil by termites and carried as microaggregates up to the surface in the next stage the microaggregates are cemented to form the nest breakdown begins when the nest is abandoned and disintegrates by splash and runoff producing colluvium or is demolished by ants in the final stage this material is integrated in the soil of the mound soil aggregates produced by splash ants and soil forming processes have different size characteristics but the original microaggregates produced by the termites are preserved in the matrix throughout it is assumed that they owe their stability to impregnation with saliva or other body fluid excreted by the termites termites produce also microaggregates in the form of fecal pellets but these appear to be unstable and are found only in the voids of the soil developed during the last stage

a new facies model for the upper gypsum of sicily italy chronological and palaeoenvironmental constraints for the messinian salinity crisis in the mediterranean the upper gypsum unit of the caltanissetta basin sicily records the last phase of the messinian salinity crisis comprising the so called lago mare event a new facies analysis study recognizes nine to ten depositional cycles consisting of seven rhythmically interbedded primary gypsum bodies and two to three sandstone bodies separated by marly terrigenous horizons showing laterally persistent vertical organization a basal thin gypsum bed is overlain by a cluster of five thicker gypsum bodies a marly interval containing two distinct sandstone horizons separates this cluster from the overlying uppermost seventh gypsum body the terrigenous arenazzolo formation in turn followed by the lower pliocene trubi formation is considered here to form the uppermost part of the upper gypsum unit the rhythmic alternation in the sandy marls and gypsum sandstone bodies records the response of sediments from shelfal to deltaic systems to precession driven arid wet climate fluctuations causing cyclical changes of both base level and water concentration during wet climate phases at insolation maxima marl and sandstone were deposited in a hypohaline environment as suggested by i the typical lago mare faunal assemblage and ii the negative 18o values during arid phases at insolation minima the reduced meteoric supply recorded by higher 18o values in the carbonate caused the development of a negative hydrological budget leading to evaporite precipitation at a basinal scale the upper gypsum unit unconformably overlies a mainly clastic evaporite unit containing carbonate breccia the so called calcare di base and or clastic gypsum towards the basin centres where the basal contact becomes conformable a primary gypsum cumulate horizon is present this layer is interpreted as a possible lateral equivalent of the halite unit present only in the deepest depocentres based on astronomical calibration of the depositional cycles the upper gypsum unit including the arenazzolo formation spans the interval between 5 33 and 5 53 ma this new age calibration allows the deposition of the halite unit to be dated between 5 6 ma top of the lower evaporites and 5 55 ma base of the upper evaporites corresponding to isotopic stages tg12 and or tg14

chemical physical and micromorphological properties of termite mounds and adjacent soils along a toposequence in zona da mata minas gerais state brazil article i nfo little is known about the effects of neotropical mound building termites in soil chemical and physical properties the influence of soil termite activity on soil characteristics was studied by assessing chemical physical and micromorphological properties of a toposequence of latosols oxisols soil samples were collected from the walls and inner parts of termite mounds and also from adjacent soil a high diversity of termite genera was found in the mounds along the toposequence together with the inquiline termites and other soil dwelling arthropods chemical analyses showed that ph and the contents of organic c and n p ca and mg were significantly higher in termite mounds compared with adjacent areas with an inverse trend for al content significant differences in ph and exchangeable al were observed between soil and mound across the slopes the mound density across the landscape was higher at the upper slope segment followed by the hill top middle slope and lower slope segments considering a lifespan of 30 years and dimensions of termite mounds found in the toposequence we conclude that the textural and chemical uniformity of latosols may be increased following the pedobiological turnover during mound building with local rates varying from 2 1 to 7 5 m 3 ha 1