neogene and quaternary history of vegetation climate and plant diversity in amazonia the neotropical amazonian and andean plant diversity developed mainly during the tertiary in amazonia miocene oral diversity seems considerably higher than today during the neogene tropical taxa entered newly created montane area and montane taxa entered the tropical lowlands the general decrease of temperature during the upper neogene and especially during the quaternary glacial periods may have caused considerable extinctions in the lowlands representation of pollen of apparently principally montane taxa podocarpus hedyosmum in miocene pliocene and quaternary sediments of amazonia and surroundings is still di cult to interpret in terms of temperature decrease at low elevation changes in precipitation may have profound impact on the composition of vegetation communities ilex and melastomataceae increase signi cantly in many glacial pollen records increase of weinmannia in amazonian pollen records seems the best indicator of downward migration of montane vegetation belts a temperature lowering at sea level of 4 5 13c during the last glacial maximum lgm seems reasonable it may have caused a downslope migration of some 700 m of the lower montane vegetation belt lower montane arboreal species may have been able to grow in higher elevation areas 500 m of amazonia increasing background pollen values of montane taxa in the area di erence between a cool and wet middle pleniglacial 60 28 ka bp and a cold and dry upper pleniglacial 28 14 ka bp thus including the lgm is evident in andean and amazonian records statements about environmental conditions of the ice age amazon should be speci ed chronologically the middle pleniglacial is a time of accumulation of uvial sediments the upper pleniglacial is a time of incision of the rivers in their sediments sedimentation started again in the lateglacial since ca 13 ka bp and the holocene when lake levels rose again based on simpli ed considerations of precipitation changes and evaporation we estimate that lgm rainfall may have been reduced by values of ca 45 10 amazonian and cordilleran lakes dried up dry rain forest was locally replaced by savanna savanna forest or cerrado type vegetation dry rain forest savanna forest and pure savanna was locally replaced by extensive semi desert dune formations lower rio branco area in present day central amazonia the present day centers of higher rainfall 2500 mm surrounded by areas of lower rainfall are refuge areas of the very wet rain forest and of the very high plant diversity 300 plant species per 0 1 ha and they should have been that equally or more during the dry climate intervals plant diversity of drier forests is in the order of 100 150 species per 0 1 ha both extinction and speciation in isolation under precipitation and temperature stress may have taken place in these refugia 2000 elsevier science ltd all rights reserved

understanding amazonian fluvial rias based on a late pleistocene holocene analog fluvial rias are elongated lakes at tributary mouths that can reach dozens of kilometers in length constituting one of the most remarkable features in the amazonian landscape thus far definitive data which documents the genesis of fluvial rias have not been published the main goal of this work was to integrate morphological sedimentological and chronological information in order to characterize fluvial paleorias in the interfluve of the purus and madeira rivers and discuss the most likely hypothesis for their genesis these paleorias were first observed through remote sensing imagery as several elongated and interconnecting belts of open vegetation that are in sharp contact with the surrounding dense forest the belts are branched and form a dendritic pattern similar to many modern drainage networks the sedimentary record of these belts revealed the prevalence of sharp based sandstones and mudstones arranged into fining upward successions which are compatible with deposition within channels active channel and abandoned channel deposits were recognized these are topped by continuous mudstones related to rapid channel abandonment and formation of a low energy basin or ria environment radiocarbon dating of these deposits recorded only late pleistocene and holocene ages ranging from 21 547 22 285 cal yr bp to 5928 6124 cal yr bp this chronology for sediment deposition is not compatible with the hypothesis of amazonian rias being formed by fluvial erosion during the last glaciation maximum low sea level with sediment accumulation during the subsequent holocene transgression instead the studied paleorias record previous tributaries of the madeira river that became abandoned as the position of this river shifted southeastward and its interfluve tilted northward inverting the drainage systems therefore a neotectonic origin of some amazonian paleorias seems most likely this hypothesis should be considered in further investigations aiming at understanding the origin of numerous modern fluvial rias that typify the amazonian landscape copyright 2014 john wiley sons ltd

late quaternary fluvial terrace evolution in the main southern amazonian tributary abstract late quaternary sedimentary deposits from amazonian lowlands are important geological archives for understanding the origin evolution and controls of the largest fluvial drainage basin on earth climate has been most often claimed as the main factor affecting river evolution in this region an increasing volume of publications have also highlighted tectonics as a relevant control this work investigates the late quaternary evolution of the madeira river the biggest southern tributary of the amazonas basin and a major waterway in south america with the goal of discussing the factor with highest potential of influence on its development in space and time the approach consisted of carrying out a detailed morphological sedimentological and chronological characterization of terrace deposits three terraces were recognized which record sandy pebbly channel muddy channel oxbow lake point bar floodplain as well as crevasse channel crevasse splay and levee deposits the topographically highest terrace t1 formed in the time interval before 43 500 and 31 696 32 913 cal yrs bp the intermediate terrace t2 between 25 338 26 056 and 14 129 14 967 cal yrs bp and the lowest terrace t3 between 12 881 13 245 and 3158 3367 cal yrs bp these terraces are the testimony of successive downcutting and sediment aggradation episodes of terrace erosion and deposition cannot be fully accommodated within the framework of fluctuating river base level tied to late pleistocene holocene global or regional climate fluctuations changes in tectonic subsidence rates might have played an important role in the equilibrium state of this particular fluvial system

amazonia during the last glacial abstract new data on the vegetational history and quaternary geology of amazonia permit an improved reconstruction of past environments in amazonia during the last glacial period although limited data from rondonia carajas and guyana show that in certain areas savanna type vegetation and savanna forest had replaced the rain forest during the late pleniglacial ca 22 000 13 000 yr b p the amazonian forest may have been split up into one major west amazonian and several other medium size forest areas this suggests a decline in rainfall of 500 to 1000 mm a reduction of 25 to 40 temperatures may also have been 2 to 6 c 4 2 c lower than today possibly substancially influencing amazonian vegetation during the humid middle pleniglacial 55 000 26 000 yr b p rivers carried a lot of water and sediment resulting in the deposition of lower terrace sediments with one dry interruption around 40 000 yr b p carajas and katira in carajas there is evidence of dry periods ocurring at about 40 000 yr b p and during the early pleniglacial ca 60 000 yr b p rivers carried little water and incised into the low terrace sediments during the dry late pleniglacial water levels rose during the late glacial 13 000 10 000 yr b p or at the beginning of the holocene 10 000 yr b p sedimentation in and of the present inundation valleys commenced after that

styles of lode gold mineralization contributing to the placers of the indian river and black hills creek yukon territory canada as deduced from microchemical characterization of placer gold grains between 1978 and 2009 approximately 430 000 oz of placer gold were obtained from the indian river and black hills creek which equates to roughly 20 of the production for the entire yukon territory during that period the area is unglaciated exposure is poor and there are few known lode gold occurrences present the technique of microchemical characterization of placer gold grains has been applied to illuminate the style s of source mineralization and their relationship to placer gold from the klondike gold district immediately to the north a total of 2 613 placer gold grains from 22 localities were characterised in terms of the au ag cu and hg content of their alloy and associated suite of opaque mineral inclusions a combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the klondike gold type 3b 8 25 ag is the main component of the placers in lower dominion creek but is augmented and eventually replaced by type 3a gold 10 40 ag in placers in the main indian river valley probably through erosion of gold bearing veins in the valley floor type 4 gold exhibits highly variable ag which may contain hg to a maximum of 11 wt this gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides tellurides sulfotellurides and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization placer gold in drainages radiating from eureka dome exhibits various proportions of types 3 and 4 gold depending on location but type 3 gold forms the major component in black hills creek and northerly flowing tributaries of the indian river with the exception of eureka and montana creeks type 5 gold is found only in placers in the middle and lower indian river it is distinguished by slightly elevated 0 05 0 17 cu in the gold alloy together with low 5 9 ag contents inclusions of bi minerals cr bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion related source candidates for such a source include undiscovered lode occurrences associated with cretaceous age intrusions to the south of the indian river or deformed cu au mo porphyry occurrences which are known to be present in the same area this analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from eureka creek to the larger placers of the indian river is minor consequently the placer gold inventory of the indian river is primarily orogenic in origin similarly the characterization of placer gold in blackhills creek strongly suggests an orogenic source this study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern klondike goldfield this information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration

chemical and mineralogical signatures of gold formed in oxidizing chloride hydrothermal systems and their significance within populations of placer gold grains collected during reconnaissance the close association of pd with au has been previously reported in highly oxidized chloride hydrothermal systems from several regions including australia brazil poland and the united kingdom the distinctive mineralization precipitated usually as a consequence of contact with reducing environments is characterized by au alloys containing low ag pd hg cu associated with selenides tellurides antimonides arsenides placer gold grains derived from this style of mineralization inherit these chemical and physical characteristics which are shown to be distinct from those placer grains derived from other styles of mineralization the discovery of placer gold grains of this type in areas remote from known in situ mineralization initiated the present project and provided a methodology for the study of a relatively large number of occurrences data describing alloy chemistry and mineralogy of polished sections of 957 placer grains from 19 localities in the united kingdom and 1 locality in the czech republic have augmented previously published data for 303 gold grains from 10 localities in the united kingdom six generic gold alloy types have been identified based on the relative proportions of au ag pd and hg mineralogical descriptions of similar gold alloys from localities worldwide usually conform to this classification but slight deviations are noted where p t conditions differ from those previously identified in devon alloy types may occur either as discrete grains or as phases within heterogeneous grains type 1 gold defined as 99 mass au type 2 gold au pd and type 3 gold au ag alloys with selenide inclusions are the most common compositions and occur at most localities although in different proportions type 4 gold is also an au ag alloy but contains sulfide and sulfarsenide inclusions gold alloys or intermetallics containing pd hg and or pd sb alloys type 5 and cu type 6 are found only at some localities as is a variety of type 3 gold selenide inclusions which contains hg the spatial arrangement of the main alloy types within heterogeneous gold grains shows a consistent precipitation order of type 1 type 2 and type 3 independent of locality interpretation of the chemical controls on alloy precipitation indicates that o2 controls whether type 1 or type 2 gold precipitates initially the formation of type 4 gold au ag alloys with sulfide inclusions is ascribed to a fall in o2 to around the hematite magnetite buffer where hs aq replaces cl aq as the aqueous gold complex types 3 and 5 gold commonly mantle earlier alloys and show evidence for aggressive replacement of the core this texture is interpreted as evidence for influx of a more oxidizing fluid which also accounts for the abundance of selenide and telluride mineral inclusions within this gold type this approach provides a plausible explanation of the complex heterogeneity within and between gold grains from the localities and accounts for the common sequence of alloy precipitation without invoking multiple episodes of hydrothermal activity to account for different alloys however the absence of thermodynamic data describing the au cu au ag cu and au ag pd hg systems prevents interpretation of the significance of some alloy types this study has demonstrated that gold mineralization associated with oxidizing strata is more widespread than currently appreciated the methodology described here not only permits identification of gold from this style of mineralization but allows initial evaluation of the economic potential of the source through the study of the alloy types this information may be gained at an early stage in the exploration process from gold grains collected during routine reconnaissance

holocene palaeoenvironmental reconstruction in norteastern brazi inferred from polen charcoal and carboon isotope records soils in the barreirinhas region maranhao state were sampled for 613 c analysis and buried charcoal fragments in the soils were radiocarbon dated three soil profiles collected in forested areas around the lagoa do caqo and one in a woody savanna mixture of non arboreal and arboreal species located approximately 10 km southeast of the lagoa were studied a high resolution pollen record was obtained from lake sediments showing that forest vegetation was predominant in the area in the early holocene from approximately 10 000 14c yr bp the pollen spectrum gradually changed suggesting the dominance of open savanna communities these were transformed to a more forested landscape woody savanna from approximately 7500 yr bp the lake sediments also record evidence of fire indicated by buried charcoal particles at several soil depths during the holocene the 613 c analysis of soil organic matter som indicates that from between approximately io0000 yr bp and 9000 yr bp to 4000 yr bp a woody savanna

geomorphological approaches to the study of neotectonics the study of morphotectonics is concerned with the analysis of landforms whose form or origins have been affected by neotectonic activity traditional morphotectonic studies have been used as a basis for more refined e g statistical analyses after the 1960s however there emerged new techniques and new approaches to the study of morphotectonics these have made more precise not only the recognition of morphotectonic features but have also improved their dating the time has come to integrate morphotectonic studies more fully both with the approaches used by other disciplines and with modern geomorphological theory

gold and sulphide minerals in tertiary quartz pebble conglomerate gold placers southland new zealand abstract auriferous quartz pebble conglomerates qpc formed during tertiary sedimentary recycling in the waimumu district southland new zealand these sediments contain fine grained gold of detrital origin with abundant surface textures and gold forms associated with authigenic gold remobilisation most authigenic gold contains no detectable silver and occurs as overgrowths on detrital au ag and au ag hg alloys that contain up to 13 wt ag and 9 wt hg fine grained au ag and au ag hg alloys are compositionally heterogeneous exhibiting both well defined silver depleted and silver enriched rims rare coarse au ag alloy is intergrown with quartz and is homogenous discrete grains of authigenic porous sheet like gold occur in carbonaceous mudstone within a qpc sequence some qpc contain abundant sulphide minerals some of these sulphides pyrite and arsenopyrite are of long distance detrital origin presumably from the otago schist whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin derived from the erosion of qpc and underlying tertiary sediments there has also been authigenic deposition of sulphide minerals in the qpc themselves these diagenetic sulphides include framboidal and anhedral marcasite and framboidal and euhedral pyrite sulphur isotope data for the sulphide minerals range from 45 to 18 relative to vcdt sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from 9 to 1 and are most likely derived from the otago schist to the north both framboidal and anhedral marcasite have lower values both the gold and diagenetic sulphides from the belle brook qpc are compositionally similar to gold and sulphides from archaean qpc porous sheet like authigenic gold is morphologically similar to gold associated with carbonaceous material in the witwatersrand in addition southland marcasite textures resemble the rounded and banded pyrite in witwatersrand qpc placers there is abundant evidence from these tertiary qpc in southern new zealand for sedimentary transport of sulphide minerals and post depositional sulphide mineralisation in the surficial environment despite an oxygen rich atmosphere these young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers many of these textures are similar to those commonly ascribed to metamorphic processes in archaean auriferous qpc

mid late pleistocene osl chronology in western amazonia and implications for the transcontinental amazon pathway abstract the origin of the transcontinental amazon drainage system remains unrevealed sedimentary deposits formed from the neogene in the amazonas and solimoes basins constitute natural archives for reconstructing this event in space and time however paleoenvironmental and chronological analyses focusing on these deposits or even their basic mapping are still scarce to allow such investigation in this context primary interests are fluvial strata related to the lithostratigraphic ica formation mapped over a widespread area in western amazonian lowlands although long regarded as plio pleistocene in age this unit has not yet been dated and its overall depositional setting remains largely undescribed the main goal of the present work is to contribute for improving facies analysis and chronology of these deposits approaching an area in southwestern amazonia and another in northern amazonia which are located more than 1000 km apart despite this great distance the sedimentological and chronological characteristics of deposits from these two areas are analogous hence facies analysis revealed paleoenvironments including active channel abandoned channel point bar crevasse splay and floodplain which are altogether compatible with meandering fluvial systems similarly optically stimulated luminescence osl dating revealed thirty three ages ranging from 65 4 16 9 to 219 6 25 1 ky in addition to three outliners of 54 0 7 6 337 3 36 9 and 346 6 48 6 ky and nine 97 1 9 9 to 254 8 23 8 ky for the areas in southwestern and northern amazonia respectively these data lead to establish that deposits mapped as ica formation over a vast area of western brazilian amazonia have a mid late pleistocene age rather than the previously inferred plio pleistocene age it follows that if plio pleistocene deposits exist in this region they remain to be dated and must be restricted to a narrow belt in western amazonia as well as isolated occurrences underlying the mid late pleistocene strata characterized herein the combination of data from this work with previously published provenance studies supports main andean sediment sources only in the mid late pleistocene it is proposed that before this time the amazon river was restricted to eastern amazonia being separated from western amazonian drainage basins due to the presence of the purus arch erosion and or subsidence of this geological feature would have promoted the connection of these drainage systems ultimately with the expressive record of the transcontinental amazon pathway into the atlantic ocean in the mid late pleistocene

late pleistocene glacial forest of humaita western amazonia abstract glacial aged vegetation dynamics of the humaita western brazilian amazonia were studied by pollen sedimentary facies 14 c dating 13 c org and c n molar two sediment cores were taken to a depth of 10 and 8 m from areas covered by grassland and dense open forest respectively the deposits represent a succession of sediment accumulation in active channel 42 600 cal yr b p abandoned channel floodplain 42 600 to 39 000 cal yr b p and oxbow lake sedimentary environments 39 000 cal yr b p to modern the predominance of mud sediments depletion of 13 c org and decrease in c n molar values identify the lake establishment in these settings low energy subaqueous conditions were developed locally favoring preservation of a pollen assemblage representing herbaceous vegetation some modern taxa from amazonia and cold adapted plants from the andes represented by alnus 2 11 hedyosmum 2 17 weinmannia 0 18 podocarpus 0 4 ilex 0 4 and drymis 0 1 at least between 42 600 and alnus with other cold adapted plants from the andes during the late pleistocene indicates that alnus probably penetrated the western amazonia lowland or was growing closer to the study site due to cooler temperatures during glacial times the present study presents the first report of a glacial age forest containing alnus in areas of the brazilian amazonian lowlands in addition to its palaeogeographical importance this work demonstrates the effectiveness of using a combination of proxies for reconstructing sedimentary environments associated with vegetation

late holocene tectonic influence on hydrology and vegetation patterns in a northern amazonian megafan abstract the factors that control the heterogeneous canopy of the amazonian rainforest have long been a topic of interest for research among all hypotheses changes in landscape due to geological processes have been increasingly defended large areas of open vegetation in the northern amazonian lowlands are confined to paleolandforms created by the abandoning of megafan depositional systems previous investigations related the megafan sedimentation in this region to late pleistocene holocene tectonic reactivations however the influence of neotectonics on both the amazonian megafans and associated vegetation cover remains to be further investigated we studied a depression preliminarily noticed in the south central sector of one amazonian megafan i e virua megafan aiming to determine if its genesis was due to recent tectonic subsidence after the megafan abandoning the investigation combined morphostructural hydrological and remote sensing reflectance analyses as well as characterization of floristic communities in the modern and ancient environments based respectively on field inventories and palynological data the results indicated a nearly 10 km long and rectangular shaped area having straight margins paralleling nne ssw trending regional structural lineaments hydrological analysis suggests that this area experiences the largest floods during wet seasons a condition compatible with the topographic data that revealed a smoother gradient of 3 m comparing this area with others from the megafan plain in the studied depression numerous forest patches are arranged as sets of parallel straight lines trending consistently in the ne sw direction the patches have spectral values that differ significantly from other forest patches over the megafan surface being compared to those of seazonally flooded forests such as igapos and varzeas that surround the megafan paleolandform in addition a forest patch within the depression revealed trees physiologically adapted to tolerate submersion 6 to 7 months per year which is also a characteristic of the inundated forests external to the megafan these data altogether led us to conclude that the studied depression consists of a shallow subsiding basin formed by tectonic reactivations in the mid late holocene tectonic instability at around 2 cal kyr bp further disturbed this system by creating ne sw trending lakes and leading to the replacement of seazonally flooded forests by grasslands thus tectonic activity in a relatively recent geological time must be accounted when analyzing plant distribution in the amazonian wetlands

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

luminescence dating of quaternary sediments recent advances duller geoffrey 2004 luminescence dating of quaternary sediments recent advances journal of quaternary science 19 2 pp 183 192 rae2008

lake patterns as related to neotectonics in subsiding basins the example of the ucamara depression peru abstract two northeast trending elongated lakes from the andean foredeep ucamara depression south maranon basin are analyzed in relation to basement structures neotectonic and scismotectonic data from the surrounding areas lake puinahua 25 60 km lies over pre cretaceous basement faults and lake punga 9 35 km which subsided in a historical times lies over a paleozoic basement uplift p axes from earthquake focal mechanisms located east and west of the ucamara depression are parallel to the lakes the trend of lineaments together will neotectonic data from the craton border suggest that lakes develop along extensional faults the western limit of the lakes are lie over folds from the front of the sub andean zone comparison of elongated lakes from the ucamara depression with other lakes from the andean foredeep suggests close relationships between lake pattern and geodynamics ria lakes are located along the front of the upper foothills and are possibly related to subsidence due to tectonic overloading of the sub andean border elongated lakes from the ucamara depression are situated in front of the wide low foothills of the sub andean thrust and fault belt subsidence is related to block faulting and tectonic overloading is weak these results are compared with previous studies of rectangular lakes from the beni basin which appear to be related to the bending of the craton border analysis of lake pattern is suggested as a complementary approach for the neotectonic study of active subsiding areas

microchemical studies of placer and lode gold in the klondike district yukon canada 2 constraints on the nature and location of regional lode sources alloy compositions and associated suites of opaque mineral inclusions have been used to define the micro chemical signature of 3 050 placer gold grains from 34 localities and 561 gold grains from six lode localities from the northeastern central and southern klondike gold district yukon canada these data augment those from a detailed parallel study of placer lode gold relationships in the western part of the klondike goldfield see chapman et al 2010 gold compositions have been considered in association with chemical controls on alloy compositions to establish whether variation between signatures represents zonation within a single mineralizing event or discrete episodes of mineralization three main gold types have been identified each of which exhibits subtypes type 1 gold is generally a simple au ag alloy with low ag 12 25 and an inclusion suite dominated by pyrite and simple base metal sulfides type 2 gold is generally ag rich 18 55 and contains hg 9 type 3 gold 10 40 ag hg 4 is prevalent in the south of the region analysis of large sample sets has allowed us to delineate the geographic extent of each gold type and their relative importance within the placer populations type 1 gold is the major component of placer deposits in bonanza hunker dominion and upper sulphur creeks type 2 gold is relatively trivial but contributes to placer gold in bear creek and some of the paleoplacers in white channel gravel in the north of the study area type 3 gold is actively augmenting the placer inventory in the south of the region this study builds upon a detailed parallel study in the western klondike which concluded that types 1 and 2 gold were emplaced by a single hydrothermal system of limited geographic extent centered on the lone star ridge here we propose that small rich hydrothermal systems were also present in the upper reaches of bear and last chance creeks and hunker dome if correct this model shows that economically important orogenic gold mineralization can be formed in smaller hydrothermal systems than previously recognized in addition type 3 gold is widespread in the south of the study area where it appears to be present at the surface consideration of the abundance of the various gold types contributing to the placers has established type 1 gold to be the most important and originally present in geographically constrained vertically persistent mineralization we propose that this type offers the best target for exploration in the northern and central klondike in the southern klondike the large placers containing only type 3b gold suggest a potentially important undiscovered source in the catchment of gold run creek

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

neotectonics and tree mortality in a forest ecosystem of the negro basin geomorphic evidence of contemporary seismicity in the intracratonic brazilian amazonia abstract neotectonic activity was increasingly recorded over the amazonian lowlands even during historical times of a few tens of centuries however detailed analyses linking structures and styles of deformation are still few considering the large dimension of the region such events in a relatively recent time are expected to have caused conspicuous impacts on amazonian ecosystems and forest disturbance changes in topography and hydrology by fracturing faulting and folding during either subsidence or uplifting are direct effects on landscape that leave marks on the structure of the current forest such as tree mortality and community succession two rectangular elbow like shaped wetlands from the left margin of the middle negro river valley in northwestern amazonia were attributed to neotectonics remote sensing imagery was applied to test this hypothesis and also provide a tectonic model that can explain deformation dynamics in this area the studied wetlands and adjacent river systems display various morphostructural anomalies compatible with a tectonic control as well as lineaments paralleling main nw and ne trending regional structures the geomorphic analysis led to suggest that the wetlands are depressions formed by nw and ne trending master boundary faults of horizontal displacements intercepted by various subsidiary faults transtensional strike slip regime is recorded by both left and right lateral faults with the wetlands corresponding to subsiding areas owned to conjugate strike slip faults habitat fragmentation and other ecological processes promoted by tectonic deformation would have impacted the overlying forest canopy by changing its structure due to tree mortality the complex compartmentalization imposed by active tectonics would have exposed the ground to contrasting hydrological conditions which controlled the rate of tree mortality within the wetlands we pose that the tectonic disturbance and associated tree mortality documented in the middle negro river evidence contemporary seismicity within the intracratonic brazilian amazonia

the application of microchemical analysis of alluvial gold grains to the understanding of complex local and regional gold mineralization a case study in the irish and scottish caledonides terranes of the appalachian caledonian orogen north of the iapetus suture host gold mineralization in north america scotland and ireland to date commercial exploitation has been confined to the appalachian terrane of canada in scotland and ireland regional analysis of the types of gold mineralization has been handicapped by the relatively small number of bedrock gold occurrences available for study however there are a large number of alluvial gold localities a technique of microchemical characterization of gold grains has been applied to facilitate the classification of alluvial gold localities in terms of the style or styles of source mineralization thereby permitting a more complete interpretation of regional gold mineralization the technique of microchemical characterization used in this study classifies populations of gold grains from each locality in terms of the assemblage of opaque mineral inclusions enclosed within the gold grains and the concentration of minor alloying elements in the gold a total of 2 400 gold grains collected from 44 sites in ireland and scotland have been studied the microchemical signatures of populations of gold grains extracted from bedrock samples correlate closely with those from adjacent alluvial populations and furthermore our data are compatible with the descriptions of the vein mineralogy of bedrock mineralization reported by other workers this result has facilitated informed speculation on the type of source mineralization of other alluvial occurrences where the bedrock source remains undiscovered populations of alluvial gold from individual localities have been grouped into larger regional data sets on the basis of similarities in microchemical signature the signatures of these data sets were analyzed using the same criteria and 10 classes of gold were identified regional analysis of the distribution of the gold classes showed that the occurrence of the four major classes correlates strongly with host terrane irrespective of geographical location gold from the southern uplands terrane is characterized by inclusions of base metal sulfides and sulfarsenides with the silver content of the gold being less than 15 percent in contrast populations of alluvial gold from the grampian and highland terranes commonly exhibit median silver contents of over 20 percent and frequently contain silver telluride and bismuth telluride inclusions which are extremely rare in gold from the southern uplands terrane however in other cases the style of mineralization is controlled by specific host or source rocks and is independent of the terrane two examples are gold mineralization within porphyry systems and gold associated with red bed sediments the study has confirmed the value of the technique of gold grain characterization using information from both concentrations of alloying elements and inclusion assemblages as a powerful tool in interpreting complex local and regional gold mineralization

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

the relationship between placer gold particle shape rimming and distance of fluvial transport as exemplified by gold from the klondike district yukon territory canada a semiquantitative study of various physical features of approximately 2 700 gold particles from 21 lode deposits and 36 placer deposits in the klondike district yukon territory canada was undertaken the shape parameters of roundness and flatness were determined using a classification method there appears to be a systematic variation between these parameters and between the parameters and the distance of transport of the particle these relationships were confirmed by quantitative measurements on critical samples rim characteristics were also estimated they show a systematic relationship between one another with the particle shape and less clearly with the distance of transport gold particle shape data shows a smooth well defined relationship to distance of transport from the lode source of the gold both roundness and flatness show a rapid increase within the first 5 km from the lode source and a slower more linear increase beyond this relationship seems to apply worldwide the relationship between the shape of a gold particle and the rim characteristic supports the conclusion that rims are formed by the removal of ag hg and cu and not by the precipitation of au thus rim thickness and the associated high fineness is probably the result of the dynamic process of ag removal in stagnant sediments to create the rim and abrasion in active sediments to remove the rim the composition of the rims represents the stable composition of gold in normal surficial environments together these observations place limits on the occurrence of new gold hammering is the main cause of shape change in fluvially transported gold particles its principal effect is to flatten particles abrasion has its most marked effect on surface texture hammering and abrasion both decrease the mass size and increase the roundness of the gold particles it is therefore possible to make a connection between hydraulic conditions during gold particle transport and its shape change

astronomical forcing of contrasting rainfall changes in tropical south america between 12 400 and 8800 cal yr b p abstract today precipitation over tropical south america is largely controlled by the seasonal movements of the inter tropical convergence zone itcz during the summer the itcz is shifted southward due to the warming of the continent paleoclimate data from southeastern amazonia and the central andes indicate that these two areas evolved similarly during the last 30 000 yr however between 12 400 and 8800 cal yr b p eastern amazonia received substantial moisture whereas the bolivian altiplano was arid this suggests that the itcz during summer was then farther north than it is today

applying dem srtm for reconstructing a late quaternary paleodrainage in amazonia abstract remote sensing is a particularly invaluable tool that has helped the detection of paleomorphologies produced by river dislocation in a variety of landscapes which has contributed in reconstructing the geological evolution of many fluvial systems this technique might provide useful information to discuss the evolution of large fluvial systems in special those located in areas of difficult access where the acquisition of field data is difficult application of remote sensing for paleodrainage characterization in densely vegetated tropical areas is scarce in the literature this work records processing of the digital elevation model dem derived from the shuttle radar topography mission srtm which succeeded in revealing an ancient drainage complex of the madeira river one of the main amazonas tributaries where other remote sensing products failed the detection analysis of this paleodrainage and of its modern counterpart within the geological framework available for this region leads to propose that activity along pre existent faults during the latest quaternary would have promoted the southeastward dislocation of a nearly 200 km long segment of the madeira river during this process an impressive paleodrainage network was left behind which was only able to be detected using the dem srtm application of this technique might be of great help to the detection of paleodrainage morphologies in densely vegetated areas similar to the amazonas lowland the dynamics of channel migration in this and many other large scale tropical river systems might benefit from the investigation based on data derived from dem srtm

geodynamic environment of quaternary morphostructures of the subandean foreland basins of peru and bolivia characteristics and study methods abstract the quaternary evolution of western amazon basins is considered using morphostructural analysis of fluvial belts and lakes meandering rivers from the basin surface are analyzed using three scales of elements the ridge and swale pattern the mosaic pattern and the fluvial belt lake patterns are differentiated according to their shape the hydrological environment and the structural context influences of basement structures and neotectonic deformations when known are considered the morphostructural patterns are discussed in the context of a balance of internal tectonic and external hydraulic and sedimentologic processes special attention is focussed on the effect of neotectonics which is generally neglected on the formation and the location of quaternary morphostructures the adaptation of study methods and sampling to the characteristics of the different morphostructures is considered

the role of tectonics in the late quaternary evolution of brazil s amazonian landscape tectonic reactivation has long been suggested to have occurred over several areas of the amazonian lowland in the neogene holocene numerous landscape changes documented in this region particularly variations in fluvial dynamics and morphologies have nevertheless been exclusively attributed to climatic fluctuations minimizing the effect of tectonics over the neogene holocene evolution of the amazon basin can result in a distorted interpretation of the sedimentary record and as a consequence equivocal paleoclimatic reconstructions climate has often prevailed as a hypothesis to explain changes in fluvial dynamics possibly due to the still scarce synthesizing publications focusing exclusively on the neotectonic influence over this region the present work provides a review of existing data focusing on quaternary tectonics over a large area of the amazonian lowland the existing information is complemented by original morphostructural data on quaternary terrains from this region which were acquired through remote sensing the goals were to analyze the geographic extent of late quaternary deposits and to determine the impact of tectonics on their development and on the latest evolution of the amazon drainage basin the analysis shows that 1 the amazonian lowland experienced geographically widespread sedimentation in the late pleistocene holocene 2 sedimentation was promoted by the creation of new depositional sites mostly likely resulting from tectonic activity and 3 tectonic reactivation impacted the latest evolution of this entire region with particular influence on drainage basins as revealed by river courses with an abundance of morphostructural lineaments evidencing fault control hence the nw ne nearly n s and to a lesser extent w e morphostructural trends recorded in all areas investigated here are generally associated with strike slip deformation in addition they are conformable with the main orientation of tectonic structures from adjacent crystalline rocks of the precambrian basement they also match tectonic trends documented in neogene and younger sedimentary deposits from this and several other areas of northeastern brazil furthermore some are detected locally in the subsurface by geophysical data furthermore a profusion of morphostructural anomalies is imprinted on the surface of neogene holocene deposits of the amazonian lowland the main ones consisting of 1 highly asymmetric modern drainage basin still undergoing organization with anomalous patterns i e trellis sub dendritic sub parallel and rectangular that often vary from one another frequent river captures and rivers flowing in contrary directions along the time and straight channels that form triangular shapes or triple junctions or with local meandering and subtle enlargements 2 an abundance of channels floodplains lakes and entire valleys with straight and orthogonal margins that locally form rhombic shapes or which may be laterally displaced 3 fluvial rias i e rivers that became enlarged as their mouth were barred bounded by sharp and orthogonal lineaments which result in rectangular shapes 4 termination of megafans into rhombic shaped depressions bounded by straight lineaments or their lateral displacement several kilometers apart and 5 rivers with asymmetric terraces laterally displaced by faults these observations together are consistent with a landscape controlled by neotectonic activity the modern seismogenic record which reveals even high magnitude earthquakes indicates active deformation over the amazonian lowland at the present time the neotectonic history of the amazonian lowland is explained within the context of an overarching mechanism most likely related to intraplate stresses this was probably caused by reactivation of pre existing structures since the neogene that was most likely promoted by activity along oceanic fracture zones and transform faults of the brazilian equatorial margin as well as tectonism in the andean region

response of alluvial rivers to slow active tectonic movement alluvial rivers respond to valley slope deformation caused by active tectonics in various ways depending on the rate and amount of surficial deformation and on the type of river on the basis of experimental results and field examples hypothetical models of river response to anticlinal uplift and synclinal subsidence were developed for different types of alluvial rivers an experimental braided channel responded to anticlinal uplift across the channel with degradation and terrace formation in the central part of the uplift with subsidence aggradation in the central reach was the main response transverse bars developed downstream of the subsidence axis an experimental meandering channel responded to slope steepening with a sinuosity increase bank erosion and point bar growth occurred downstream of the anticlinal axis and upstream of the synclinal axis upstream of the uplift axis and downstream of the subsidence axis where the slope was flattened water flooded over bars local convexity in longitudinal profiles of the middle rio grande new mexico is considered to be formed by a domal uplift local aggradation and degradation could be explained by the effect of uplift the san joaquin river california which is now highly controlled does not show clear adjustment to the rapid subsidence due to ground water withdrawal it shows however a channel pattern adjustment to active tectonic subsidence that has been occurring for a long time the san antonio and guadalupe rivers in texas both increase their sinuosity significantly where monoclinal movements steepen valley slopes

native gold from the kamenka barabanovsky and kharuzovka alluvial placers northwest salair ridge western siberia russia typomorphic features and possible bedrock sources abstract northwest salair ridge is one of the oldest alluvial gold mining regions it is located in the western part of the altai sayan folded area the placers localized along the kharuzovka and kamenka barabanovsky creeks the right branch of the suenga river were chosen for investigation the only bedrock deposit in the study area is the novolushnikovsky gold deposit located in the suenginsky placer field of the egoryevsky district the source of gold supply for the kharuzovka and kamenka barabanovsky placers spatially related to the hypothetical continuation of the type ii ore zones of the novolushnikovsky deposit the composition of gold grains derived from placers was determined based on a representative collection a distinctive feature of native gold from type ii ores is unusually high content of hg up to 20 wt the composition of gold grains from studied placers shows that the average hg content is 1 5 5 wt however there is a group of hg rich placers

a tectonically triggered late holocene seismite in the southern amazonian lowlands brazil abstract definitive criteria for identifying the triggers of soft sediment deformation structures ssdss from sedimentary deposits are still under construction in this work we aimed to contribute to advancing this issue describing the sedimentological characteristics of an interval of ssdss formed in a region of the amazonian lowlands having numerous morphostructural evidences of neotectonic reactivation the deformed interval is 1 5 m thick and consists of load and flame structures interbedded with massive beds sandwiched between well stratified deposits of a fluvial sandy channel succession with a radiocarbon age of 1301 1382 cal yr bp the ssdss are intercepted by numerous small scale normal graded faults the most interesting is that these faults display segmented nw se trending planes that propagate upward after horizontal displacements of a few millimeters processes intrinsic to channel dynamics could not justify this fault morphology which is related to recurrent short lived seismic perturbations with intervening sedimentation following a main tremor a relation to fault reactivation is proposed with basis on the fault trend compatible with the regional structural pattern and the numerous morphostructural anomalies verified in the study area including trellis and sub trellis drainage rectangular and to a less extent annular channels anomalously enlarged channels orthogonal channels and valleys channel captures sudden changes in channel morphology moderate to high basin asymmetry factor af between 10 and 27 moderate to high transverse topography symmetry factor t 0 3 0 5 and anomalous longitudinal river profiles with concavity values 11 to 30 indicative of rivers in process of adjustments the morphology of the fault graded beds described herein might constitute a feature of relevance for identifying seismites in other fluvial deposits

the role of late pleistocene holocene tectono sedimentary history on the origin of patches of savanna vegetation in the middle madeira river southwest of the amazonian lowlands abstract savanna patches are features of the amazonian landscape that have been long under intense debate but there are still questions about the main factors that have determined their establishment and evolution within the rainforest matrix in particular their geological substrates were poorly documented the aim of this work is to reconstruct the tectono sedimentary history of four savanna patches in the middle madeira river southwest amazonia and discuss its potential control over the development of the savanna communities the approach consisted in the integration of geomorphological sedimentological chronological and floristic data the results revealed that the savanna substrates are sandy deposits aged between 118 9 and 35 6 ky overlain by late pleistocene to holocene muddier successions

principles of drainage basin analysis from multisource data application to the structural analysis of the zaire basin abstract classifications which attempt to combine descriptive and genetic aspects of types of drainage basins must take into account the application of new techniques satellite imagery high precision topographic maps and digital terrain models in addition the use of new concepts such as neotectonics and drainage basin history will lead to a reassessment of classification schemes after a review of current classifications we propose a genetic scheme that is based on the development of drainage networks and which includes integration of local and regional scale perturbations state of the art methods of drainage basin analysis are applied to the zaire basin to provide new information on deep structures the influence of basin and block tectonics is revealed in the context of major late proterozoic shear faults which have been rejuvenated up to the present day this is demonstrated from an analysis of the drainage network in the light of geological and geophysical observations

microchemical studies of placer and lode gold in the klondike district yukon canada 1 evidence for a small gold rich orogenic hydrothermal system in the bonanza and eldorado creek area orogenic gold camps are commonly associated with placer deposits although in many cases the relationship between placer and lode source may be unclear placer gold deposits in bonanza and eldorado creeks in the northern klondike district goldfield yukon canada probably yielded more than 2 million ounces moz from a small geographic area but known occurrences of gold bearing orogenic style veins from which the placers were derived accounted for only a few thousand ounces the alloy chemistry and mineralogy of opaque mineral microinclusions microchemical signatures within these gold grains were compared to that of samples collected during a detailed placer gold sampling program collection of placer samples from the headwaters of watercourses in this unglaciated area allowed identification of microchemical signatures within gold populations from proximal sources which augmented information from lode gold sources to facilitate characterization of local gold mineralization a total of 1 163 gold grains from 18 lode sources and 2 777 grains from 27 placer localities within the bonanza and eldorado creek drainages were characterized in terms of the ag and hg content of the alloy together with the associated inclusion suite and two main compositional ranges along with several subtypes were identified type 1 gold is a simple au ag alloy but various subtypes have been identified on the basis of different ranges of ag and or specific inclusion assemblages type 2 gold is an au ag hg alloy composition the spatial arrangement of the gold types shows a systematic change in microchemical signature the lowest ag gold occurs in the lone star ridge area and the ag content increases both in the northeast and southwest the appearance of te bearing mineral inclusions in populations coincides with increasing ag and argentite inclusions are present only in high ag gold toward the periphery of study area type 2 gold is absent at lone star ridge and all localities to the east but is progressively more abundant in sample localities to the west the geographic distribution of gold types correlates well with placer abundance inferred from the production from previous mining activity and the study area is bounded by creeks and gulches in which there was insufficient placer gold for economic extraction we conclude that the lone star ridge area represents the epicenter of a single hydrothermal system in which spatial and temporal fluid evolution has generated compositional zonation variation in ag is ascribed to decrease in au ag aq following preferential au deposition in the center of the system controls on hg contents within au alloy are unclear but may be a consequence of mineralization temperature the complex but consistent signatures of placer gold samples from the eldorado creek drainage represents liberation of types 1 and 2 gold grains which coexist in some lode occurrences the different proportions of types 1 and 2 gold observed in placer and lode samples from west of lone star ridge reflect both temporal and spatial evolution of the hydrothermal system the main eldorado creek placers exhibit a signature most similar to lode gold from the buckland and nugget zones on lone star ridge placer gold in upper bonanza creek is distinguished by a relatively narrow ag range and the small but consistent presence of hessite in the inclusion suite no corresponding lode occurrence for this gold type has been identified to date this is the first time that the compositional signatures of major placer gold deposits derived from orogenic gold vein systems has been studied in this detail and the application of basic chemical principles to fluid evolution and electrum precipitation has allowed demarcation of an area in which extremely rich gold mineralization was formerly present while simultaneously identifying the most promising areas for further exploration for undiscovered lode gold

geological and geochemical controls on the silver content fineness of gold in gold silver deposits abstract a compilation of microprobe fineness data and a re evaluation of bullion production fineness data suggest that the gold fineness model of fisher 1945 can be updated and applied to a wide range of gold mineralised environments the major deposit classes are characterised by the overall average or range of deposits averages and total range of gold fineness values as follows archaean 940 780 1000 slate belt 920 800 1000 plutonic 825 650 970 porphyry 700 1000 650 1000 volcanogenic 650 850 520 870 epithermal 440 1000 0 1000 the archaean including witwatersrand slate belt and plutonic classes are characterised by a high and consistent average fineness and a narrow fineness range there is no clear subdivision within these classes according to deposit style or inferred genesis the porphyry volcanogenic and epithermal classes have a variable average deposit fineness and a wide total range of fineness values within these three classes higher fineness is typical of deposits in andesitic rather than rhyolitic volcanic settings deposits with cu au rather than cu mo or pb zn element associations and deposits with telluride or selenide minerals solubility calculations indicate that in all these environments gold and silver form the same aqueous species with agcl 2 dominating for silver and au hs 2 dominating for gold in all the environments the fluids transport an equal amount of gold and silver in the form of their bisulphide complexes therefore the observed gold fineness values can not be explained by the transporting mechanism only low silver in the ores and higher gold fineness values observed in the archaean plutonic and slate belt environments can be attributed to sulphidation as the dominant mechanism of ore deposition sulphidation of wall rocks destabilises only the bisulphide complexes of gold and silver in contrast epithermal volcanogenic and porphyry environments are characterised by a diversity of ore depositing mechanisms cooling boiling mixing which act alone or in combination and destabilise chloro and bisulphide complexes of gold and silver the high silver concentration in the ores low fineness values and large variation in the fineness values in these environments can be attributed to the complexities in the ore depositing mechanisms

observational evidence for alternate modes of track altering binary tropical cyclone scenarios abstract an observational study of western north pacific tropical cyclones tc revealed many cases of two tcs whose tracks were altered by processes that were quite different from the mutual advection fujiwhara type processes thus four conceptual models are proposed to describe these track alterations a conceptual model called direct interaction is proposed that is a modification of one by lander and holland and has three modes 1 a one way influence in which the track of a smaller tc that is embedded in the circulation of a larger tc has a cyclonic orbiting motion but no significant track alteration of the larger tc is apparent 2 a similar case in which a mutual advection occurs with the tracks of both the smaller and larger tcs being altered and 3 a subset of 2 in which the mutual advection includes an attraction component such that the two similarly sized tc circulations eventually merge into a larger circulation with a single center during the 7 yr period 1989 95 the one way influence

new geological framework for western amazonia brazil and implications for biogeography and evolution abstract although many of the current hypotheses to explain the origin and distribution of the amazon biodiversity has been based directly or indirectly on geological data the reconstruction of the geological history of the amazon region is still inadequate to analyze its relationship with the biodiversity this work has the main goal to characterize the sedimentary successions formed in the brazilian amazon in the neogene quaternary discussing the evolution of the depositional systems through time and analyzing their main controlling mechanisms in order to fill up this gap radar image interpretation sedimentological studies and radiocarbon dating allowed the mapping of plio pleistocene to holocene units along the solimoes amazonas river brazil this integrated work led to the characterization of five sedimentary successions overlying miocene deposits of the solimoes pebas formation which include the following ica formation plio pleistocene deposits q1 37 400 43 700 14c yr b p deposits q2 27 200 14c yr b p deposits q3 6730 2480 14c yr b p and deposits q4 280 130 14c yr b p these deposits occur mostly to the west of manaus forming nw se elongated belts that are progressively younger from sw to ne indicating a subsiding basin with a depocenter that migrated to the ne the reconstruction of the depositional history is consistent with significant changes in the landscapes hence the closure of a large lake system at the end of the miocene gave rise to the development of a plio pleistocene fluvial system this was yet very distinct from the modern drainage with shallow energetic highly migrating braided to anastomosed channels having an overall northeast outlet this fluvial system formed probably under climatic conditions relatively drier than today s during the early pleistocene there was pronounced erosion followed by a renewed depositional phase ca 40 000 14c yr b p with the development of prograding lobes and or crevasse splays associated with a lake system i e fan delta and or fluvial flood plain areas after a period of erosion a fluvial system with eastward draining channels started to develop at around 27 000 14c yr b p the fluvial channels were overflooded in mid holocene time this flooding is attributed to an increased period of humidity with a peak between 5000 and 2500 14c yr b p the data presented herein support that rather than being a monotonous area the amazonia was a place with frequent changes in landscape throughout the neogene quaternary probably as a result of climatic and tectonic factors we hypothesize that these changes in the physical environment stressed the biota resulting in speciation and thus had a great impact on modern biodiversity