climatic periodicity during the late pleistocene from a loess paleosol sequence in northwest argentina abstract a 42 m thick loess paleosol sequence was studied in the pre andean intramountain valley of tafi del valle 100 km west of the city of san miguel de tucuman at an elevation of 2280 m asl in northwest argentina the sequence contains 28 paleosols alternating with loess layers the loess layers are coarse loamy and have large hexagonal polyhedra the soil layers are restricted to bt horizons having fine loamy textures prismatic structures and organo argillans on the structural surfaces surface a and eluvial e horizons are absent from the whole sequence soil truncation is unlikely as no remains of these horizons or erosional unconformities were identified the dated part of the sequence encompasses a time span of 10 080 years from 17 580 bp at 5 2 m depth to 27 660 bp at 42 3 m depth the base of the exposed sequence the youngest strata are probably missing either because of a depositional hiatus or because of post depositional erosion as dated paleosols in adjacent areas indicate that loess deposition continued during at least the early holocene the 20 bt c couplets identified in the dated sequence correspond to 20 climatic oscillations in 10 080 years thus every 500 years on average there was a climatic change from dry cool conditions with loess deposition to moist warm conditions favouring soil development this might have been related to recurrent northward shifts of the polar front accompanied by weakening of the mid latitude south pacific anticyclone similar short term periodicity has been identified in the ice cap of greenland and in tropical lake level records so such frequent climatic changes during the late pleistocene seem to have been global in character

trace element characteristics ree patterns and partition coefficients of zircons from different geological environments a case study on indian zircons abstract twelve zircon separates from diverse geological settings in peninsular india have been analyzed for their zr hf sc ree th and u contents employing neutron activation techniques the whole rock samples of acid charnockite and a related rock of syenitic composition were also analyzed for these elements after separation of zircons the study reveals that 1 a the zircons exhibit a wide range of variation in elemental abudances and their ratios 2 b their chondritic normalised ree patterns exhibit negative or positive ce and negative eu anomalies in addition to smooth hree enriched patterns without anomalies and 3 c there are significant differences in the ree partition coefficients of zircons from the charnockite and the associated syenite the overall elemental abundances and ree patterns of the zircons could be explained in terms of crystallochemical factors bulk compositional differences of parent melts and paragenetic histories

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

geochemistry of shales from the archean 3 0 ga buhwa greenstone belt zimbabwe implications for provenance and source area weathering abstract phyllites from the archean 3 0 ga buhwa greenstone belt zimbabwe were deposited on a stable cratonic platform analyses of the phyllites generally define a single geochemical group based on major and trace element abundances the phyllites are strongly depleted in cao na2o and sr with respect to average archean upper crust by contrast k2o ba and rb are enriched several times relative to average archean upper crust reflecting basin scale k metasomatism transition metals are somewhat depleted whereas high field strength elements are typically enriched relative to average archean upper crust samples generally have fractionated lree patterns average cen smn 2 4 with small negative eu anomalies average eu eu 0 78 and generally flat hrees average gdn ybn 1 3 such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite mixing calculations which massbalance the rees and th sc suggest 70 tonalite 15 mafic volcanic rocks and 15 granite as possible proportions for the source of the phyllites a nearby early archean 3 5 ga continental nucleus preserves similar lithologies and is the probable source intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95 100 nearly complete depletion of cao and na2o and high al2o3 na2o ratios fine grained sediments of comparable age and presumed tectonic setting elsewhere in southern africa show similar geochemical characteristics implying that source area compositions and weathering intensities were similar these mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at 3 0 ga across southern africa

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

rare earth element and mineralogic changes in holocene soil and stream sediment a case study in the wet mountains colorado u s a size fractions of soil and stream sediment developed on plutonic metamorphic or mixed plutonic metamorphic sources in the wet mountains colorado have been analyzed for ree rare earth elements and mineralogy the clay sized fractions 0 5 m and 0 5 2 m or 2 m have ree contents that are similar to more enriched than the source eu sm ratios that are similar to the source and la lu ratios that are higher than their corresponding source the 0 5 m fractions have lower ree contents than the corresponding 0 5 2 m fraction in every stream sediment for example unweathered san isabel batholith near the sampled stream sediment contains ree 511 712 ppm and eusmratio 0 17 0 19 and lalu cn 9 8 10 3 the corresponding 0 5 and 0 5 2 m fractions of sediment formed from san isabel batholith are enriched in the ree ree 1135 and 2225 ppm respec are slightly lower in eu sm ratios 0 16 and 0 14 respec and are higher in la lu ratios 12 2 and 16 3 respec than the unweathered rocks the silts have lree light ree contents and eu sm ratios very similar to the source due to similar mineralogy but silts have hree heavy ree contents higher than the source due to high zircon for example silt sized fractions in stream sediment draining the san isabel batholith contain ree 808 ppm eusmratio 0 18 and lalu cn 7 4 in contrast the sands contain lower ree contents similar to higher eu sm ratios and lower la lu ratios than the corresponding source due to depletion in heavy minerals concentrated in the ree and containing lower eu sm ratios relative to quartz and feldspar depleted in the ree and containing higher eu sm ratios for example the sand fraction from stream sediment from the san isabel batholith has ree 206 ppm eusmratio 0 17 and lalu cn 5 6 small variations of accessory minerals concentrated in the ree like allanite or sphene may cause drastic variation in ree content in the sand fraction clays and silts derived only from the plutonic rocks have higher ree and similar to lower eu sm ratios than corresponding fractions derived only from migmatites e g ree of silt fractions from migmatites 224 ppm and from plutons 375 808 ppm eu sm ratios from migmatites 0 17 and from plutons 0 11 0 18 the ree content and the eu sm ratio of clays and silts derived from migmatites are much more similar to the average of mcps mid continent platform sediment than corresponding fractions derived from the plutons mcps has ree 192 ppm eusm 0 19 lalu 8 0 the eusm ratio of sands derived from plutons and migmatite are lower to mostly higher than the mcps the ree content of sands from plutons are not necessarily higher than the mcps due to variation in heavy minerals like allanite or sphene concentrated in the ree sediments derived from mixed sources have ree contents in fine fractions that may not necessarily reflect the ree content of the nearest source for example 0 5 2 m and silt fractions developed on the san isabel batholith with a migmatite upstream have the high ree content of the batholith ree in 0 5 2 m fraction 1100 ppm silt 573 ppm while the corresponding 0 5 m fraction has the very low ree content of the migmatites ree 110 ppm these results suggest fine grained material near the source may sample a much larger area than coarser material near the source thus analysis of fine grained sediment associated with coarse grained material near the source may give a much different impression of the source than would determination of the source using classic petrographic examination of the sand or gravels

a comparison of glsl irsl and tl dating methods using loess deposits from japan and china abstract a comparison of ages of loess samples from japan and china was made utilizing three methods thermoluminescence tl using quartz fine grains green light stimulated luminescence glsl using quartz fine grains and infrared stimulated luminescence irsl using polymineral fine grains the glsl and irsl ages of the three samples agreed within the error ranges however the tl ages are greater than those of glsl and irsl the glsl ages of the samples were concordant with independent age control given by tephrochronology but the tl ages were overestimated the degree of overestimation varied with age and location of the samples which might be connected with age and sediment source differences and the bleaching environment of loess in east asia

trace elements in shale as indicators of crustal provenance and terrane accretion in the southern canadian cordillera in this study the trace element geochemistry of shale is used to infer the nature of adjacent crustal blocks during terrane amalgamation and accretion in southern british columbia the geochemistry of about fifty shale samples from one of the best dated stratigraphic sections in the north america cordillera is used to 1 infer proximity to source regions characterized by juvenile ocean arcs continental arcs and ophiolites and 2 address a controversy concerning the timing of terrane amalgamation and accretion this study includes analyses of shale from the entire mesozoic stratigraphic section of the cadwallader terrane and overlying tyaughton basin as well as argillites from the structurally adjacent bridge river complex the mid cretaceous albian strata of the upper taylor creek group show light rare earth lree enrichment la n sm n of 3 0 and anomalously high cr and ni older units upper triassic lower cretaceous that stratigraphically underlie the upper taylor creek group have less lree enrichment high ba n la n ratios and no cr or ni anomaly the mid cretaceous strata were probably derived from a continental arc during a period of uplift and erosion of ultramafic rocks probably ophiolites and the co extensive nature of these sediments suggests accretion of many basement terranes to the edge of a continent the trace element geochemistry of the older strata is similar to that of sediment derived from an oceanic arc however lower cretaceous strata of the relay mountain group may record the incipient collision and progressive closure of this ocean basin the bridge river complex which is dominated by oceanic rocks and is interpreted to be an accretionary complex contains two suites of argillites one of which shows significant lree enrichment and isotopic data suggest a jurassic cretaceous age for these argillites that compose the matrix in this complex leitch et al 1991 we suggest that the bridge river complex was a jurassic cretaceous accretionary complex that formed outboard of the methow section of rocks and was not juxtaposed with the cadwallader terrane and tyaughton basin until the albian when these terranes were accreted to north america paleomagnetic data from these rocks maxson et al 1993 indicate that accretion took place at the latitude of central mexico and that subsequent strike slip faulting brought these terranes to their present position by eocene time

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

grain size characteristics and provenance determination of sediment and dissolved load of alaknanda river garhwal himalaya india alaknanda river is a major tributary of river ganga in its upper catchment area the significant feature of the river is its high rates of sediment erosion and chemical denudation the alaknanda river catchment area has many major hydroelectric power projects and is always in the news because of environmental concerns in the present study both suspended and bank sediment characteristics of the river were evaluated for grain size and composition the results show that suspended and bank sediment sizes vary from 8 79 to 56 34 m and 23 73 to 563 24 m respectively suspended sediments are mostly finer to symmetrically skewed whereas the tributaries are found to be negatively skewed in case of bank sediments grains are moderately sorted and mesokurtic the provenance determination using rare earth elements and major oxide composition in suspended and bank sediments shows the higher himalayas as the dominant supplier of sediments along with the sediments the source reservoir of dissolved load was determined using the water composition and an often used forward model the results of forward model show silicate and carbonate weathering processes as a major contributor of dissolved load using this multi approach of dealing with sediment and water geochemical nature of water and sediments of the alaknanda river has been better understood from the present study

mossbauer spectroscopy analysis of iron containing minerals in the chinese loess chinese loess has been analyzed in bulk using mossbauer spectroscopy in order to investigate the effects of climate on the overall iron containing mineralogy strongly weathered loess is found to differ from weakly weathered loess in two significant respects it contains a much larger content of fine grained superparamagnetic hematite and the ratio of paramagnetic fe 3 fe 2 is substantially increased the suggestion is that weathering results in the liberation of fe from silicate minerals and the formation of fine grained hematite measurements of total iron content by atomic absorption spectrometry highlight that the strongly weathered loess has a higher content of iron than the weakly weathered loess and that this is too large an effect to be explained by the leaching of carbonate the indication is that the mineralogy of the dust input to the loess plateau varies as a function of climate one possibility is that the grain size and hence mineralogy of the dust fluctuates in response to vegetational cover in proximal source areas during interglacial times the vegetational cover would expand to stabilize these proximal sources resulting in a bias to finer dust from more distant sources thus the mechanisms linking climate and mineralogy in the chinese loess may be more complex than previously thought

chemical and mineralogical comparison of the pennsylvanian fountain formation colorado u s a an uplifted continental block to sedimentary rocks from other tectonic environments the mineralogic and chemical composition of composites of cobble and pebble sized rock clasts clay stones siltstones fine sandstones and size fractions of two sandstones of the fountain formation near beulah colorado were determined the sandstones are arkosic and most plot in the basement uplift or transitional continental fields of qfl diagrams the mineralogic composition of the sandstones is similar to that of the composite of rock clasts at a given location al mg fe ti mn sc th ree and cs are concentrated in the clay minerals or heavy minerals and are diluted by quartz or feldspar consequently they are inversely correlated with si cr hf k rb ba and sr are poorly correlated with si cr is concentrated in clay and in heavy minerals in the silts k rb ba and sr are correlated with k feldspar mica or clay mineral abundances sedimentary rocks from the fountain formed from an uplifted continental block the element concentrations plot in the fields of active or passive continental margins in many tectonic discrimination diagrams probably as a result of derivation from granitoid sources of similar composition the ree patterns eu anomaly size k and rb concentrations of the fine sandstones are mostly similar to those of the corresponding composite of rock clasts at a given location suggesting that these parameters might be the most reliable in distinguishing sources the most reliable discrimination diagram for uplifted tectonic blocks like the fountain may be a plot of k2o vs eu anomaly size since both of these parameters were very similar in concentration in both the sedimentary rocks and presumed source of the fountain most sedimentary rocks in this study have larger negative eu anomalies than those from the nasc north american shale composite or mcps midcontinent platform sediment this suggests a more evolved source for the fountain than that for average platform sediment

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

the pedosedimentary evolution and chronology of tortugas a late quaternary type site of the northern pampa argentina new micromorphological magnetic susceptibility geochemical and geochronological data have been collected from tortugas a key late quaternary type site of the northern pampa argentina thus enabling the detailed reconstruction of the pedogenic and sedimentary history of this loess paleosol sequence paleosols developed in the ceres formation and the overlying lower member of the tezanos pinto formation retain evidence for leaching bioturbation and particularly clay translocation processes being dominant both paleosols are truncated evidence for water sorting and sediment reworking as well as continued bioturbation and redistribution of carbonates suggests that intervening depositional phases were complex the previously recorded hypsithermal soil developed in the upper member of the tezanos pinto formation has similar characteristics to the buried paleosols yet does not appear to have been truncated at this site and is only covered by a thin layer of sediment san guillermo formation this latter unit has been pedogenically welded onto the hypsithermal soil thus creating the present day surface soil complex a series of optically stimulated luminescence dates from throughout the sequence provide a stratigraphically consistent chronological framework ca 1 150 ka for the pedosedimentary reconstruction yet conflict with the previously reported thermoluminescence based chronology for the site and region ca 1 90 ka

geochemistry of renuka lake and wetland sediments lesser himalaya india implications for source area weathering provenance and tectonic setting the geochemical investigation of sediments deposited in the renuka lake basin and its adjoining wetland has shown variation in the distribution and concentration of major trace and rees the major elements are depleted in the lake in relation to wetland and that of post archaean australian shale paas except for cao which is strikingly in excess and has a dilution effect on sio2 and other oxides and trace elements the wetland sediments on the other hand are enriched in al2o3 fe2o3 k2o and tio2 and the latter three show a positive correlation with al2o3 in both wetland and lake sediments suggesting their association with phyllosilicates and similar source rocks the enrichment of y zr ni th u and nb in wetland compared to lake and their similarity with paas in the former suggests more clay fractions in the wetland a high zr hf ratio in wetland and lake sediments and a positive correlation of zr with y and hree indicate zr control on hrees however higher zr yb and zr th ratios in wetland compared to lake indicate mineral sorting during the process of lighter particles clays being trapped in wetland soil this is also reflected from negative correlation of gdn ybn with al2o3 and a strong positive correlation with sio2 in wetland sediments the wetland in this context has a control on lake sediment chemistry the chondrite normalized ree patterns are essentially the same for lake as well as wetland sediments but abundance decreases in the former the similarity of pattern with that of paas and negative eu anomaly indicates a cratonic source of sediments in a plot of the individual samples wetland samples cluster while lake samples are separated indicating fractionation of lake sediments a strong positive correlation of lan ybn with al2o3 and a positive correlation of zr lree and zr lan ybn suggest that lrees are controlled by both phyllosilicates and zircon the chemical index of alteration cia indices in lake sediments and in wetland are higher than paas indicating moderate chemical weathering in the source area the petrography lack of felsic magmatic rock fragments and negative correlation between zr gd yb c indicate sedimentary source rocks for the detritus this is in conformity with the lesser himalayan sedimentary sequence belonging to neo proterozoic proterozoic age and constituting lake catchment of renuka the tectonic delineation and discriminant function plots of lake and wetland sediments indicate their cratonic and or quartzose sedimentary orogenic terrain source that has been deposited in a passive margin setting

major and trace elements of stream sediments from the lowermost amazon river geochemical results from river bottom sediments taken from the lowermost amazon river are presented bedload sediments were analyzed for 10 major and 38 minor elements mud bulk sediments exhibit a tendency to a slight enrichment in cr mn rb sr zr cs ba and hf pointing to the importance of these elements in the silt fraction generally cu zn rb cs ba and sn are clearly associated with clay minerals while y zr hf th and u are associated with heavy minerals the location of this area in an intensive chemical weathering environment results in the strong modification of sediment composition chemical weathering processes strongly modify original sediment composition and can be recognized apart from provenance processes on the basis of their geochemical signature discriminant analysis makes the detection of less marked differences in sediment characteristics possible thus improving the accuracy of distinctions between groups the amazon mouth and xingu mouth xm are discriminated principally by the textural maturity of their sediments the jari mouth jm shows highest co and fe2o3 contents despite similarities between the amazon north branch anb and amazon south branch asb the anb can be differentiated on the basis of its higher sr p2o5 and na2o contents the asb on the basis of its higher zn mno and cao contents and the estreitos de breves est based on its higher abundance of ni and cu higher contents in p2o5 fe and mn are related to laterite covers

glacial interglacial variations in the neodymium isotope composition of seawater in the bay of bengal recorded by planktonic foraminifera this study presents neodymium isotope and elemental data for cleaned planktonic foraminifera from odp site 758 in the southernmost reaches of the bay of bengal in the north east indian ocean cleaning experiments using oxidative reductive techniques suggest that diagenetic fe mn oxyhydroxide coatings can be effectively removed and that the measured nd isotope composition reflects the composition of seawater from which the foraminiferal calcium carbonate was precipitated modern core top pulleniatina obliquiloculata and globorotalia menardii give end values of 10 12 0 16 and 10 28 0 16 respectively indistinguishable from recent direct measurements of surface seawater in this area a high resolution nd isotope record obtained from g menardii for the past 150 kyr shows systematic variations end 3 on glacial interglacial timescales the timing of those variations shows a remarkable correspondence with the global oxygen isotope record which suggests a process controlling the nd isotope composition that responds in phase with global climate cycles palaeoclimate reconstruction indicates that during the last glacial maximum changes in monsoon circulation resulted in a reduction in rainfall over the indian subcontinent and a decrease in the flux of river water delivered to the bay of bengal thus changes in the riverine input of nd a change in either flux or composition most likely caused the isotope variations although changes in dust source or local ocean circulation may have also played a role these results clearly establish a link between climate change and variations in radiogenic isotopes in the oceans and illustrate the potential of nd isotopes in foraminifera for high resolution palaeoceanographic reconstruction

another look at rare earth elements in shales linear correlation and mass balance considerations indicate that clays are more important than zircon or other heavy minerals in hosting both light and heavy ree in cratonic shales considering the uniformity of the eu anomaly and ree distributions in cratonic shales it seems likely that detritus is thoroughly mixed during weathering and sediment transport and that the ree patterns in shales reflect the average ree pattern of the sources regardless of the ratio of clays to heavy minerals in the shales shales suspended river loads and near shore marine sediments all have broadly similar ree patterns and reported hree depletions in the latter two groups may reflect incomplete dissolution of heavy minerals during sample preparation ree distributions in shales and suspended river loads determined by inna are similar to those in average upper continent crust and ree distributions in shale averages nasc and paas seem to be reasonably representative of the average composition of post archean upper continental crust

sr and nd isotopic and trace element compositions of quaternary volcanic centers of the southern andes isotopic compositions of samples from six quaternary volcanoes located in the northern and southern extremities of the southern volcanic zone svz 33 46 s of the andes and from four centers in the austral volcanic zone avz 49 54 s range for 87sr 86sr from 0 70280 to 0 70591 and for 143nd 144nd from 0 51314 to 0 51255 the ranges are significantly greater than previously reported from the southern andes but are different from the isotopic compositions of volcanoes in the central and northern andes basalts and basaltic andesites from three centers just north of the chile rise trench triple junction have 87sr 86sr 143nd 144nd la yb ba la and hf lu that lie within the relatively restricted ranges of the basic magmas erupted from the volcanic centers as far north as 35 s in the svz of the andes the trace element and sr and nd isotopic characteristics of these magmas may be explained by source region contamination of subarc asthenosphere with contaminants derived from subducted pelagic sediments and seawater altered basalts by dehydration of subducted oceanic lithosphere in the northern extremity of the svz between 33 and 34 s basaltic andesites and andesites have higher 87sr 86sr rb cs and hf lu and lower 143nd 144nd than basalts and basaltic andesites erupted farther south in the svz which suggests involvement of components derived from the continental crust in the avz the most primitive sample high mg andesite from the southernmost volcanic center in the andes 54 s has sr and nd isotopic compositions and kr b and ba la similar to morb the high la yb of this sample suggests formation by small degrees of partial melting of subducted morb with garnet as a residue samples from centers farther north in the avz show a regionally regular northward increase in sio2 k2o rb ba ba la and 87sr 86sr and decrease in mgo sr kr b rb cs and 143nd 144nd suggesting increasingly greater degrees of fractional crystallization and associated intra crustal contamination

a multi proxy study of argentina loess marine oxygen isotope stage 4 and 5 environmental record from pedogenic hematite abstract a 12 m loess paleosol section la angostura from tafi del valle tucuman argentina was studied using rock magnetic measurements geochemistry mossbauer effect particle size and nd isotopes in order to elucidate changes in paleoenvironment at the site combining the results of these experiments allowed us to separate the effects of the two major controls parent material and pedogenesis on the measured parameters two behavior regimes were identified in the sediment 1 in the lower half of the section pedogenic processes though weak dominate the variation of the sediment s physical properties and 2 for the upper half of the section subtle changes in parent material control the majority of the measured parameters even those typically controlled by pedogenesis e g clay fraction applying rock magnetic mossbauer and bulk geochemistry measurements in concert we calculated an iron budget for the loess 80 of the iron in the sediment is in paramagnetic minerals 20 of the iron is in hematite 1 6 of the sediment s mass and 1 of the total iron is present as magnetite maghemite 0 1 of the sediment s total mass neither the pedogenic formation nor the depletion of ferrimagnetic material with increasing pedogenesis was consistently observed in this sediment throughout the entire section the concentration of hematite is argued to be sensitive to in situ alteration and it is further surmised to be sensitive to the available moisture at the site finally we present a curiosity within the error of our chronology the concentration of hematite seems to correlate with the coeval climate record from antarctica vostok it is possible that the sedimentary magnetic record from la angostura is a proxy for the strength of the south american summer monsoon

imaging of the moho depth in madagascar through the inversion of gravity data geodynamic implications several e w profiles of bouguer anomalies across the precambrian basement were inverted by applying a linearized inversion procedure in the spectral domain and using a simplified two layer model with a crust upper mantle interface of constant density contrast from these inverted profiles a contour map of moho topography has been constructed which covers the area of exposed precambrian basement in the eastern and central parts of the island such imaging of the moho depth in particular exhibits a n20 e trending zone of substantially thinned crust along the axis of the island this parallels the east coast margin which resulted from the northward motion of india relative to madagascar in the upper cretaceous from combined analysis of geological tectonic and metamorphic observations this prominent crustal feature in the precambrian basement of madagascar is interpreted as the relic of continental lithospheric extension and thinning possibly related to panafrican collapse tectonics on the eastern edge of the mozambique belt this extensional feature would have been reactivated only in part during the rifting and dislocation stages of the madagascar and india continental blocks as evidenced by an axis of cretaceous volcanism emphasized by the ankaratra and itasy massifs final separation nevertheless occurred further east at the site of the east coast margin

dominants and accumulation of rare earth elements in sediments derived from riparian and depressional marshes the rare earth elements rees in the sediments of the xianghai wetlands were measured by inductively coupled plasma spectrometry the rees accumulation rates in two sedimentation cores derived from the riparian and depressional marshes were determined by 210pb method the results showed that rees concentrations in the xianghai wetland sediments rees 116 mg kg 1 were lower than the corresponding values in chinese soils 181 mg kg 1 and river sediments rees 158 191 mg kg 1 under alkaline conditions with ph 8 2 10 3 the light rees were more enriched than the heavy rees cerium is the predominant element and accounts for 30 33 of the total rees rees in the depressional marsh sediments were relatively high rees 127 vs 104 mg kg 1 especially light rees contents a significantly positive correlation was found between the neighboring elements except pr and dy the different types of vertical distribution of rees between the riparian and the depressional marsh can partly result from long term differing hydrological regimes generally depressional marsh had accumulated much more rees than riparian marsh the mean accumulation rates of rees 102 98 vs 48 89 g cm 2 year 1

a new method for the determination of ceiii ceiv ratios in geological materials application for weathering sedimentary and diagenetic processes abstract relative abundances of rare earth elements ree in geological materials are used widely to investigate diverse geochemical issues such as the origins of igneous rocks or the degree of stratification of water columns in oceans one of the ree cerium ce can exist in either trivalent or tetravalent forms depending on the redox condition thus knowledge of the oxidation state of ce in rocks and minerals could potentially be used to constrain the redox states of past and present geological environments however the use of ce for this purpose has been hampered by an inability to measure its oxidation state directly here we present a new method employing the x ray absorption near edge structure for making such determinations in samples with ce concentrations as low as 15 ppm by analyzing a range of diverse geological materials granites manganese nodules and cherts we showed that the method could identify significant differences in ce speciation between materials formed under widely differing redox conditions used together with the degrees of ce anomalies measured in ree patterns this information has promise for refining our knowledge of various geochemical processes such as weathering sedimentary and diagenetic processes that control the ree behavior such a refinement should in turn improve geological interpretations based upon ree data

pedogenesis and paleoclimate interpretation of the magnetic susceptibility record of chinese loess paleosol sequences in the chinese loess paleosol sequences from luochuan haanxi province china variations in the magnetic susceptibility have been accepted as an excellent proxy for paleoclimate and the standard interpretation is that climatic processes have enhanced the magnetic susceptibility of the paleosols beyond the base line level of the loess by using mineral magnetic properties and a soil chemistry extraction procedure we show that most of the magnetic susceptibility signal in both the paleosols and the loess is due to pedogenesis in effect loess deposition and pedogenesis are competing processes at all times and the presence of a paleosol simply indicates that the latter process was predominant these results imply that any interpretation of the paleoclimate record of loess paleosol sequences must recognize the pedogenic nature of the magnetic susceptibility signal and that the focus of future research must be on the critical role of climate in the development of the paleosols

geochemical signature of provenance in sand size material in soils and stream sediments near the tobacco root batholith montana u s a abstract trace element geochemistry of sandstones are being used to determine provenance we have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks six sets of first order stream sediments soils from adjacent slopes and a variety of parent rocks were collected from southwestern montana u s a sampling in a low relief area ensured that climate and residence time of soils on slopes could be eliminated as variables sand size fractions of stream sediments and soils and the corresponding parent rocks granodiorite quartz monzonite granite gneiss biotite tonalite gneiss and amphibolite were analyzed for most major elements and selected trace elements petrologic modal analysis of the parent rocks and the 0 25 0 50 mm fraction of each sand was done to monitor major mineralogic control if any on chemical compositions of the samples our data show that the abundances of the si and al in sediments do not discriminate provenance abundances of ca mg fe and ti may broadly distinguish between sands derived from metamorphic and igneous source rocks at least in the area studied differences in abundances of the ba and th and the ratio of la lu between granitic tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied however the size of the eu anomaly in the ree patterns of different daughter sediments is not diagnostic of parent rocks abundances of co and sc distinguish between sediments derived from felsic and mafic rocks a better provenance discrimination is obtained if the ratios la sc th sc la co ba sc and ba co are used petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other the chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy this is expected because these elements may concentrate in accessory minerals and or may weather out into aqueous or clay mineral fractions it is also compatible with conclusions of previous studies that some of these elements do not reside in sand size fractions of siliciclastic sediments

trace element geochemistry and sedimentary provinces a study from the tasman geosyncline australia abstract the abundances of la th u and hf have been determined in flysch sequences of the tasman geosyncline australia the arc derived graywackes differ from those of rifted continental margins and marginal basins in containing low abundances of la th u and hf la 9 2 1 7 ppm th 1 4 0 6 ppm u 0 52 0 2 ppm hf 2 1 0 6 ppm reflecting similar low abundances in the andesitic source rocks graywackes of rifted continental margins and marginal basins in contrast are mature and characterised by high abundances of la th u and hf la 39 9 9 ppm th 16 0 6 ppm u 3 40 0 5 ppm hf 7 9 ppm mirroring increased abundances in the granitic and sedimentary source rocks sedimentary rocks deposited in inter arc basins show wide dispersion but have characteristics intermediate between these two types the th u ratio for magmatic arc sediments is 2 7 for most other sedimentary rocks it remains uniform at 4 5 0 3 for sediments having andesitic and dacitic rocks as provenance the la th ratios are 6 7 and 4 5 respectively this ratio remains constant at 2 6 for most quartzose sedimentary rocks

tectonic environment of the devonian gramscatho basin south cornwall framework mode and geochemical evidence from turbiditic sandstones the portscatho formation within the allochthonous unit of the middle and upper devonian gramscatho group is a thick sequence of deep water sandstones and interbedded slates deposited by southerly derived turbidity currents into the gramscatho basin of south cornwall throughout an approximately 3 5 km thick sequence the portscatho formation is petrographically and chemically coherent except that the upper section shows a higher proportion of metamorphic clasts high but variable cr and low uniform zr abundances complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition similar to average upper continental crust but with an admixture of minor intermediate basic material flysch deposition took place in a fore arc setting the presence of an arc to the south of cornwall during the devonian implies that there was subduction at the margin of the gramscatho basin whose ultimate closure was accommodated by the northward stacking of flysch ophiolite nappes

geochemistry of precambrian and paleozoic siliciclastic rocks from the iberian range ne spain implications for source area weathering sorting provenance and tectonic setting abstract the major and trace element chemical composition of precambrian and paleozoic shales and sandstones from the iberian range spain has been investigated to determine the provenance and tectonic setting of these rocks as well as to appraise the influence of the weathering hydraulic sorting and recycling processes upon source rock signature the samples studied belong to a prograde sequence from diagenesis to anchizonal grade and are mainly composed of quartz illite mica phases and or kaolinite with chlorite feldspars and carbonates as minor components the major element distribution chemical index of alteration cia indices reflects that recycling processes have been important in homogenizing the composition of shales and sandstones the higher cia indices observed in the silurian and devonian shales 80 and 85 respectively compared with those of precambrian to ordovician and the carboniferous age about 70 indicate that their source area underwent more intense chemical weathering processes possibly due to climatic and or tectonic variations alternatively the source area of the silurian and devonian shales may have been composed of recycled sedimentary materials mineral fractionation is mainly observed in coarser rocks through zircon apatite and xenotime accumulation although the shales are not free of these phases the zircon content of the fine grained rocks is not high enough to affect rare earth element ree contents but phosphate minerals apatite and xenotime at least partially control the ree distribution the influence of apatite and xenotime are slightly higher in the precambrian shales as the phosphate concentration produces a decrease in the la sm ratios chondrite normalized ree patterns and negative eu anomaly size of the studied rocks are similar to that of post archean australian shales paas indicating that they originally come from a differentiated silicic source the higher ree contents in the studied shales in relation to paas indicate that recycling processes in the iberian range sources were probably more intense than that of paas the slight differences among the ree patterns of the different groups of shales probably do not reflect changes in source area composition but instead suggest variations in mineral sorting chemical weathering and or sediment recycling the th sc co th cr th cr v v ni ratios support a primitive silicic source for these rocks with the higher th sc and lower co th in cambrian and ordovician shales indicating a higher proportion of felsic material in their primitive source area the compositional maturity of analyzed sandstones is typical of cratonic environments and their la sc th and zr contents reflect their passive continental margin setting

composition and classification of paleozoic flysch mudrocks of eastern australia implications in provenance and tectonic setting interpretation abstract the mudrock suites of paleozoic flysch sequences of eastern australia can be characterised on the basis of the relative abundances of phyllosilicates and tectosilicates mainly feldspar and quartz a mudrock maturity index is defined as the ratio of total phyllosilicates to phyllosilicates feldspar quartz mudrocks are classified into tectic phyllo tectic and phyllic types on the basis of increasing maturity based on studied examples of eastern australia preliminary characteristics of mudrocks of various tectonic settings are identified oceanic island arc mudrocks are of the tectic type characterised by high feldspar and low phyllosilicate content they are derived from calc alkaline andesitic rocks of the magmatic arc mudrocks of the continental island arc and active continental margins andean type and strike slip basins are characterised by a higher phyllosilicate content and are mainly phyllo tectic in type passive margin mudrocks are of the phyllic type characterised by extremely high phyllosilicate and low feldspar contents suggesting their recycled nature mudrock maturity is accompanied by an increase in the k2o na2o ratio from oceanic island arc to continental island arc and active continental margin to passive margin type mudrocks

geochemistry of mansar lake sediments jammu india implication for source area weathering provenance and tectonic setting abstract the sediment geochemistry including ree of surface and core samples from mansar lake along with mineralogical investigations have been carried out in order to understand the provenance source area weathering hydrolic sorting and tectonic setting of the basin the geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters the major element log values fe 2 o 3 k 2 o vs sio 2 al 2 o 3 and na 2 o k 2 o vs sio 2 al 2 o 3 demarcate a lithology remarkably similar to that exposed in the catchment area the chondrite normalized ree patterns of lake samples are similar to post archaean australian shale paas with lree enrichment a negative eu anomaly and almost flat hree pattern similar to a felsic and or cratonic sedimentary source however the la th sc plot of samples fall in a mixed sedimentary domain close to upper continental crust ucc and paas suggesting sedimentary source rocks for the mansar detritus it also indicates that these elements remained immobile during weathering and transportation the mineralogical characteristic rees and high field strength elements hfse together with the high percentage of metamorphic rock fragments in the siwalik sandstone support a metamorphic source for lower siwalik sediments a very weak positive correlation between zr and sio 2 poor negative correlation with al 2 o 3 negative correlation of la yb n and gd yb n ratios with sio 2 and positive correlation with al 2 o 3 suggest that zr does not dominantly control the ree distribution in mansar sediments the petrographic character and textural immaturity indicate a short distance transport for the detritus the distribution of elements in core samples reflect fractionation the higher zr th and zr yb ratios in coarse sediments and paas compared to finer grained detritus indicate sedimentary sorting plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting it therefore reveals lower siwalik depositional history

rare earth element geochemistry of australian paleozoic graywackes and mudrocks provenance and tectonic control abstract in the paleozoic graywackes of eastern australia there is an increase in the total abundance of rare earth elements ree a light to heavy ree ratio and a decrease in the chondrite normalized eu anomaly with the increase in sio2 al2o3 and k2o na2o ratios this is due to the change in the dominant source rocks from andesite to dacite to granite gneiss and sedimentary rocks similarly an increase in total ree and a decrease in eu anomaly is also seen in the associated mudrocks with the increase in total clay content this correlation in paleozoic sedimentary rocks suggests that the ree characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin the oceanic island arc type graywackes are characterised by their lower total ree abundance only slight enrichment of lree over hree and the absence of a negative eu anomaly on chondrite normalised plots the continental island arc type graywackes are discriminated by their higher ree abundance and la yb ratio the presence of a small negative eu anomaly on the chondrite normalised plots and the significant enrichment of eu compared to paas post archean average australian shale the sedimentary rocks deposited on andean type active continental margins passive margins platform and cratonic basins are all characterised by the high enrichment of lree over hree and the presence of a pronounced negative eu anomaly on chondrite normalised plots and can not be discriminated from each other by ree patterns alone

geochemical differentiation of quaternary sediments from the pampean region based on soil phosphorus contents as detected in the early 20th century abstract data obtained in the early 20th century on the total phosphorus tp content in soil materials from the pampean region have been processed and plotted on maps of the region approximately 1200 subsoil data and 1400 surface horizon data obtained using the same analytical method have been processed according to differences in phosphorus content several areas have been demarcated limits and characteristics of these areas have been correlated with data from the more recent literature regarding the p distribution and with both mineralogical and sedimentological information on surface materials results suggest that a confluence of sediments from different provenances i e the andean range the pampean hills and the brazilian shield is found in the argentine pampean region

the last interglacial in the chinese loess plateau a petromagnetic investigation of samples from a north south transect abstract magnetic properties are reported for samples from the last interglacial palaeosol s1 oxygen isotope stage 5 and its underlying loess l2 at three sites representing the north wupu middle yichuan and south pucheng portions of a 560 km transect across the chinese loess plateau clp the main objective was to test the magnetic model put forward by evans and heller 1994 geophysical journal international 117 257 264 as a result of their work on the s3 l4 couplet and to check to what extent it may vary in time and space their model argues for two magnetic components an original aeolian input containing a mixture of hematite magnetite and maghemite superimposed upon which is a pedogenic component consisting of ultrafine magnetite particles we find that the model is readily applicable but that a difficulty arises at the southern end of the transect where it appears that wetter and warmer climatic conditions arrest or even reverse the magnetic enhancement typical of most soils across the clp a secondary aim was to assess the use of magnetic susceptibility to quantitatively estimate palaeo precipitation the methods of maher et al 1994 earth and planetary science letters 125 461 471 and han et al 1996 studia geophysica et geodaetica 40 262 275 do not agree and once again the southerly wetter warmer gradient leads to particular difficulties