paleopedology and stable isotope chemistry of late silurian vertic paleosols bloomsburg formation central pennsylvania abstract paleosols with vertic vertisol like features occur in repetitively stacked fining upward sequences bounded by marine erosional surfaces in the bloomsburg formation ludlovian to pridolian of central pennsylvania paleosol development was strongly influenced by repeated changes in relative sea level which resulted in subaerial exposure and pedogenesis of marginal marine sediment accumulating along a low energy mud dominated coastline in a delta plain environment bloomsburg paleosols are characterized by very weak horizonation abundant pedogenic slickensides and a micromorphology dominated by sepic fabrics and peds bounded by stress cutans pedogenic carbonate is sparse occurring principally as micrite nodules stable carbon isotope compositions of stratigraphically low pedogen c carbonate mean 13c 1 92 pdb are attributed to pedogenesis proximal to a marine coastline in stratigraphically high paleosols showing evidence of rhizolith development isotopic compositions of pedogenic carbonate suggest a significant contribution of carbon from c3 flora atmospheric p co2 levels estimated for these calcretes 4500 5500 ppmv are significantly greater than present day values low 18o values for pedogenic carbonate mean 18o 12 12 pdb record isotopic exchange between paleosol carbonate and 18o enriched fluids associated with the conversion of original soil smectites to illite and chlorite during burial diagenesis paleosols bounded by marine erosional surfaces show variable degrees of marine hydromorphism attributable to rising water table during transgression evidence for hydromorphism includes extensive reduction of iron as well as relative increases in cao mno mgo p2o5 na2o and sr at the tops of paleosols due to introduction of marine fluids and sediment into open soil fractures and burrows the abundance of vertic features in all bloomsburg paleosols by analogy with holocene vertisols indicates the recurrence of a seasonally wet dry dry 4 8 months year paleoclimate n the central appalachian region at a paleolatitude of about 20 south

the history of the major rivers of southern britain during the tertiary the evolution of the drainage system of lowland britain is discussed on the basis of available geological evidence including that from terrestrial sites and that which has more recently become available from offshore exploration of the north sea the english and bristol channels and the irish sea tertiary stratigraphy throws considerable light on landform and river development paleocene destruction of a chalk cover which seems to have been incomplete in western britain was accompanied by basin sedimentation under a tropical climate the major elements the thames solent hampshire proto avon river irish sea river and possibly an early trent river existed almost throughout the cenozoic the influence of atlantic rifting and thermal doming in nw britain appears to have been stronger and more temporally focused than the persistent flexuring that determined and maintained tertiary drainage lines in the se here also the folded mesozoic terrains on the surface contrast with the more dominant block faulted relief of the palaeozoic oldlands the rivers of the se can be shown to have extended or reduced their lengths in response to relative sea level change and gentle warping drainage antecedence the destruction of the solent system and the breaching of the english channel are also evident by contrast the major river systems of the west are now entirely submerged long term stability of the drainage pattern reflects a persistent tectonic regime in the south with a subdued low relief landscape having a weathered regolith and dense vegetation cover meandering river channels and alluvial styles predominated although channel forms varied according to sediment load slope and discharge variability coarse gravel dominated accumulations are rare and localized chemically stable lithologies dominate the clastic component throughout it is apparent that the deeply incised river valleys seen today are related to high predominantly coarse sediment yields encouraged by substantial rapid climate changes in the pleistocene this emphasizes the significance of mechanical compared with chemical weathering for the rate and nature of landscape dissection and the modifications that have arisen as a result of glaciation frost climate weathering rapidly changing climates and sea levels the stratigraphical evidence here reviewed is at variance with older largely geomorphologically based landform evolution models denudational chronology but gives considerable support to the recent proposals emphasizing the significance of paleocene erosion and enduring low relief landscapes and drainage systems evolving alongside fold development during the paleogene given the depobasin evidence now available postulated fluvially active episodes can and must be linked to contemporaneous deposition some at least of the many controversies involving the identity of erosion surfaces the dating of them using only residual deposits and weathering mantles and the selection of particular tertiary episodes as ones of landscape development can now be resolved

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

dynamic impact of oscillating sources on the north atlantic deep circulation a process study in this process study the dynamic effects of fluctuating deep water sources on the deep circulation of the north atlantic are examined by means of a 1 layer model the model is used with a realistic topography of the atlantic ocean between 65 n and 25 s and has an open southern boundary according to observations the northern inflow of deep water is subject to variability on timescales between days and decades the same holds for the deep western boundary current dwbc which acts as a link between the deep water inflow and the interior circulation the timescales of the observed variability motivate the forcing functions used in the model they determine the nature of the models response the flow response relative to the local mean flow can be considerably larger than the original perturbation at the source normalizing the perturbed flow with the local steady state flow leads to the identification of sensitive areas their location and shape are determined by the difference between the characteristic travel paths of the perturbation and the streamlines of the steady state circulation in the eastern equatorial atlantic a forcing with periods between 1 and 3 months gives rise to larger flow perturbations than a forcing with periods between half a year and 1 year forcing with the latter periods leads to a stronger response in the western equatorial atlantic and in the interior ocean changes in the direction of the deep equatorial flow are driven by low frequency oscillations of the dwbc in the model this is consistent with observations in the area perturbations radiate into the interior away from the eastern boundary as long rossby waves as this propagation is along geostrophic contours consequently large parts of the abyssal interior are inaccessible the results support the notion that the area where the dwbc encounters the equator is not only a key region for the slow interhemispheric exchange of heat and tracers but also for the transport of information by fast internal waves along the equator

origin of bermuda s clay rich quaternary paleosols and their paleoclimatic significance red clayey paleosols that are chiefly the product of aerosolic dust deposition are interbedded in the quaternary carbonate formations of the bermuda oceanic island system these paleosols provide a basis for reconstructing quaternary atmospheric circulation patterns in the northwestern atlantic geochemical analyses were performed on representative paleosol samples to identify their parent dust source fine grained fractions were analyzed by energy dispersive x ray fluorescence to determine trace element zr y la ti and nb concentrations and to derive geochemical signatures based on immobile element ratios these ratios were compared with geochemical signatures determined for three possible sources of airborne dust 1 great plains loess 2 mississippi river valley loess and 3 saharan dust the zr y and zr la ratios provided the clearest distinction between the hypothesized dust sources the low ratios in the paleosol b horizons most closely resemble saharan dust in the 2 m size class fraction contributions from the two north american loessial source areas could not be clearly detected thus bermuda paleosols have a predominantly saharan aerosolic dust signature saharan dust deposition on bermuda during successive quaternary glacial periods is consistent with patterns of general circulation models which indicate that during glacial maxima the northeast summer trade winds were stronger than at present and reached latitudes higher than 30on despite lower than present sea surface temperatures in the north atlantic

abrupt chemical weathering increase across the permian triassic boundary previous studies have suggested a variety of causes for the end permian extinction event including a bolide impact flood basalt volcanism methane clathrate dissociation or some combination of catastrophic processes one common feature of these hypotheses is the prediction of an enhanced earliest triassic greenhouse new high resolution geochemical results from graphite peak antarctica support an abrupt increase in chemical weathering in the earliest triassic among otherwise genetically similar paleosols of similar provenance relative to the latest permian paleosols the earliest triassic paleosol exhibits greater leaching greater accumulation of immobile rees and evidence of lower soil po2 with no evidence of an erosional unconformity between the paleosols which are separated by 15 cm stratigraphically these results support a rapid shift perhaps 10 000 years to an earliest triassic greenhouse and a role for methane release in the extinction event and its aftermath

permo carboniferous paleoclimate of the congo basin evidence from lithostratigraphy clay mineralogy and stable isotope geochemistry abstract approximately 1000 m of strata in the upper paleozoic lukuga formation in the dekese core in the central congo basin provide lithostratigraphic mineralogical and isotopic evidence for substantial climatic variation within a long lived lacustrine basin lithostratigraphic indicators of cold climate include polymictic strata dropstone deposits and coupled laminations of fine clay size material and coarse silt glacial varves dropstones are concentrated in three stratigraphic zones in the lower 425 m of the lukuga formation and varved strata occur in two broad stratigraphic zones in the lower 700 m of the formation these sedimentological indicators suggest that the lower 2 3 of the lukuga formation was strongly influenced by frigid conditions and glacial like processes the clay size fraction of 97 samples is dominated by detrital minerals including quartz feldspar chlorite illite and poorly ordered expansible 2 1 phyllosilicates based on variation in the mineralogy of these samples the lukuga formation is divisible into three clay mineral zones cmzs numbered in ascending stratigraphic order cmz 1 and cmz 3 include several horizons of expansible 2 1 phyllosilicates that represent warmer wetter intervals intervening cmz 2 is a long 500 m zone of chlorite and illite with no expansible phyllosilicates and is interpreted as a continuous cold frigid interval there are numerous calcite cemented layers including spar filled veins that cross depositional bedding and represent postburial alteration radiaxial fibrous cements that displace detrital grains and micrite that crystallized near the time of deposition eighty two stable isotope analyses of micrite yield 13c values that range from 44 6 to 4 1 and 18o values that range from 20 0 to 5 0 vpdb the carbon isotope data likely reflect a range of local carbon sources derived from bacterial activity and are unrelated to paleoclimatic conditions in contrast stratigraphic patterns in the oxygen isotope data suggest five or six intervals of frigid conditions conducive to glacial processes

direct observations of the current structure east of the bahamas quasisynoptic absolute velocity sections were obtained in the western north atlantic to the east of the bahamas on five cruises from 1992 to 1994 showing complex spatial and temporal variability of the currents throughout the water column lowered acoustic doppler current profilers were used on all cruises for comparison an acoustically tracked free fall profiler was used on the august 1992 and june 1993 cruises where simultaneous profiles from both methods are available depth averaged differences are less than 0 01 m s 1 in the ensemble mean and standard deviation though small the mean difference appears to be statistically non zero for one of the two doppler profilers used no cause has been found the individual velocity sections show a banded structure of currents in the offshore direction with strong baroclinic and depth averaged components extending over the full sampling domain two sections show a picture of the southward flowing deep western boundary current dwbc differing from previous observations during august 1992 and june 1993 the mean dwbc core was located more than 100 km offshore in observations prior to 1990 it was usually near 50 km offshore net southward transport below 800 m integrated from the coast to 400 km offshore varied from 15 to 50 sv with a mean of about 40 sv 1 sv 1 106 m s 1 our mean is consistent with previous estimates our sections add new evidence that the transport is highly variable to at least 400 km offshore

anthropogenic carbon accumulation in the subtropical north atlantic 1 recent data suggest the accumulation of anthropogenic carbon dioxide canth in the subtropical north atlantic is not occurring at a steady rate throughout the water column carbon measurements from three transatlantic cruises along 24 5 n in 1992 1998 and 2004 were investigated for changes in canth using both a back calculation shortcut technique and extended multiple linear regression for three time periods 1992 1998 1998 2004 and 1992 2004 we observed spatial and vertical changes in canth storage along with a general increase in total concentration in the surface layers total dissolved inorganic carbon tco2 and canth concentrations increased in line with atmospheric co2 levels tco2 8 8 0 5 mol kg 1 for 1992 1998 and 8 6 0 5 mol kg 1 for 1998 2004 and canth 8 0 0 2 mol kg 1 for 1992 1998 and 6 8 0 3 mol kg 1 for 1998 2004 in deeper waters canth was significantly different than zero for all depths above 5000 dbar between 1992 and 2004 while on a subdecadal timescale significant variability was observed for canth at a depth range of 800 1000 dbar evidence is presented for the arrival at 24 5 n at depth of freshly ventilated labrador sea water from the subpolar north atlantic between 1992 and 1998 as well as consistent smaller canth signals alongside the mid atlantic ridge this is in addition to low level stable increases identified in the deep eastern basin between 1992 and 2004 the first time that canth has been detected and confirmed by new measurements of carbon tetrachloride and cfc 11 from 2004 these results highlight the importance of the subtropics as a site for long term canth storage away from the surface

temperature and salinity variability in the deep western boundary current abstract a ten year time series 1984 1993 of repeat hydrographic sections from offshore abaco island the bahamas 26 5 n is used to define the mean and time dependent characteristics of the deep western boundary current dwbc the dwbc flow is divided into four vertical layers based on chlorofluorocarbon cfc concentration and formation regions upper layer cfc core 3 9 5 0 c second layer classical labrador sea water 3 2 3 9 c third layer cfc minimum 2 4 3 2 c deepest layer cfc core 1 85 2 4 c time series analysis of mean layer properties and their anomalies showed that the temperature and salinity of each layer did not increase or decrease monotonically with time variations in temperature and salinity were characterized by 2 3 yr period oscillations variability between years is illustrated by subtracting repeat sections of temperature and salinity along levels of both constant pressure and constant potential density to determine an original water mass modificati

reconstruction of a subalpine grass dominated ecosystem lake rutundu mount kenya a novel multi proxy approach palaeoecological reconstructions based on a single proxy are limited but by combining pollen biogeochemistry and grass cuticle analysis ecosystem structure and function can be better understood lake rutundu is a small subalpine lake on the northeast flank of mt kenya during the last glacial pollen evidence suggests a shrub grassland dominated by afroalpine taxa and poaceae representing a dry cold open environment the 13c values of terrestrial biomarkers imply a high proportion of c4 plants grass cuticle analysis allows resolution of the different c4 subtypes and shows that the vegetation was dominated by tall c4 panicoid grasses prone to frequent fires during the holocene poaceae pollen declined while subalpine shrubs increased the 13c values of terrestrial biomarkers imply a c3 dominated vegetation together with an expansion of rainforest at lower altitudes this suggests wetter conditions more favourable to c3 plants increased percentages of c3 pooid grass cuticles confirm a reduction in moisture stress

archean to recent aeolian sand systems and their sedimentary record current understanding and future prospects the sedimentary record of aeolian sand systems extends from the archean to the quaternary yet current understanding of aeolian sedimentary processes and product remains limited most preserved aeolian successions represent inland sand sea or dunefield erg deposits whereas coastal systems are primarily known from the cenozoic the complexity of aeolian sedimentary processes and facies variability are under represented and excessively simplified in current facies models which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes archean and neoproterozoic aeolian successions remain poorly constrained palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts during the triassic widespread desert conditions prevailed across much of europe during the jurassic extensive ergs developed in north america and gave rise to anomalously thick aeolian successions cretaceous aeolian successions are widespread in south america africa asia and locally in europe spain and the usa several eocene to pliocene successions represent the direct precursors to the present day systems quaternary systems include major sand seas ergs in low lattitude and mid latitude arid regions pleistocene carbonate and holocene modern siliciclastic coastal systems the sedimentary record of most modern aeolian systems remains largely unknown the majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes whereas successions representing linear and star dunes remain under recognized research questions that remain to be answered include i what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology ii what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation iii can sophisticated four dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation and iv is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of earth history when large scale aeolian systems were active or has the evidence to support the existence of other major desert basins been lost for many periods throughout earth history

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

significance of coniferous rain forests and related organic matter in generating commercial quantities of oil gippsland basin australia contrary to the conventional belief that humic coal generates primarily gas 3 billion bbl of recoverable oil has been discovered in the humic coaly succession of the fluviodeltaic latrobe group upper cretaceous tertiary that serves as both the reservoir and the source for hydrocarbons in the offshore gippsland basin of southeastern australia evidence for generation of liquid hydrocarbons from the coaly succession includes 1 similarity of n alkane distribution in the oil and in the coal extracts 2 high wax content of oil up to 27 by weight 3 high ratio of pristane phytane in oil 5 6 and 4 dominance of c29 steranes in the oil in the gippsland basin coniferous rain forests dominated by kauri vegetation flourished in a raised bog setting present temperate climate and kauri vegetation of new zealand are considered to be the modern analog to the gippsland basin the coniferous vegetation provided large quantities of hydrogen rich exinite macerals such as cutinite and resinite with potential to generate oil high rainfall raised ground water level low oxygen high acidity and low nutrient conditions of a raised bog setting were suitable for preserving organic matter a comparison of gas chromatograms of oils in the gippsland basin with gas chromatograms of oils generated by hydrous pyrolysis in the laboratory from the immature source rocks suggests that the paraffinic fraction of the oil was derived from coal and the naphthenic fraction was derived chiefly from resin

lake cyclicity as response to thermal subsidence a post camp scenario in the parnaiba basin ne brazil abstract the jurassic cretaceous transition was marked by the fragmentation of the west gondwana supercontinent and consequent opening of the atlantic ocean this event resulted in extensive lava flows in the central portion of west gondwana composing the central atlantic magmatic province in the parnaiba basin ne brazil a large lacustrine system succeeded this event nevertheless the post camp scenario and the influence of the decreasing isotherms in the mesozoic sedimentation are poorly understood in this sense cyclostratigraphy and outcrop based facies analysis were carried out in the lacustrine stratigraphic record allowing the recognition of four paleoenvironments central lake sheet like delta front lakeshore and ephemeral fluvial channels mainly organized in shallowing upward cycles the upsection transition of thin black shales and limestones to thick oxidized mudstones and stratified sandstones reflects the evolution of underfilled to overfilled lake settings the lacustrine succession is organized in four major depositional cycles characterized by millimeter to centimeter scale cycles bounded by unconformities or flooding surfaces the cycles define a retrogradational progradational retrogradational stacking pattern with random and frequent changes in lithology and cycle thickness cyclostratigraphic data suggest that the upward increasing accommodation space was controlled by the post camp thermal subsidence as well as shifts in sedimentary supply and water inflow outflow the time span of this subsidence phase lasted from even before the camp magmatism 200 ma affecting the whole deposition of the lacustrine succession and was interrupted by the cretaceous magmatic event 126 ma subsidence was also enhanced by crustal loading associated with thick subsurface jurassic intrusions and probably influenced the whole basin in consequence of flexural effects

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

construction of a palaeogene tide dominated shelf influence of top chalk topography and sediment supply wessex basin uk palaeogene siliciclastic sediments reading and london clay formations in the wessex basin record an upward change through four tidally influenced deposition systems the lowest reading formation was deposited in estuarine and coastal plain environments the overlying london clay records discrete upward changes from deltaic to mixed tidal flat to tidal sandsheet bagshot sands erosional topography on the top chalk surface had an important control on deposition 1 topographic lows controlled the initial sites of marine flooding and sedimentation 2 lows would have acted as conduits for sediment delivery 3 lows created additional accommodation by the compaction of thick mud fills 4 lows may have formed preferential sites for delta growth as topography was reduced by sediment infill depositional units tended toward more sheet like geometries heavy minerals reveal that an upward progradational aggradational trend and the development of more sand rich facies are related to a change in provenance toward sand rich cornubian sources

measuring plume related exhumation of the british isles in early cenozoic times mantle plumes have been proposed to exert a first order control on the morphology of earth s surface however there is little consensus on the lifespan of the convectively supported topography here we focus on the cenozoic uplift and exhumation history of the british isles while uplift in the absence of major regional tectonic activity has long been documented the causative mechanism is highly controversial and direct exhumation estimates are hindered by the near complete absence of onshore post cretaceous sediments outside northern ireland and the truncated stratigraphic record of many offshore basins two main hypotheses have been developed by previous studies epeirogenic exhumation driven by the proto iceland plume or multiple phases of cenozoic compression driven by far field stresses here we present a new thermochronological dataset comprising 43 apatite fission track aft and 102 u th sm he ahe dates from the onshore british isles inverse modelling of vertical sample profiles allows us to define well constrained regional cooling histories crucially during the paleocene the thermal history models show that a rapid exhumation pulse 1 2 5 km occurred focused on the irish sea exhumation is greatest in the north of the irish sea region and decreases in intensity to the south and west the spatial pattern of paleocene exhumation is in agreement with the extent of magmatic underplating inferred from geophysical studies and the timing of uplift and exhumation is synchronous with emplacement of the plume related british and irish paleogene igneous province bipip prior to the paleocene exhumation pulse the mesozoic onshore exhumation pulse is mainly linked to the uplift and erosion of the hinterland during the complex and long lived rifting history of the neighbouring offshore basins the extent of neogene exhumation is difficult to constrain due to the poor sensitivity of the ahe and aft systems at low temperatures we conclude that the cenozoic topographic evolution of the british isles is the result of plume driven uplift and exhumation with inversion under compressive stress playing a secondary role

alluvial sandstone composition and paleoclimate i framework mineralogy abstract systematic variations in compositional maturity of first cycle fluvial sandstones are found in the cutler formation permian and fountain formation permian pennsylvanian in colorado and in the gondwana supergroup permian triassic of peninsular india these variations reflect changing climate during deposition climate is considered to be the critical factor affecting maturity because source rocks granite granite gneiss and tectono environmental setting alluvial systems associated with basement cored block uplifts were similar and remained relatively unchanged throughout deposition of all three of the units the roughly 3 200 m thick gondwana succession consisting of five sandstone petrofacies is characterized by the following sequence of change oldest to youngest in compositional maturity expressed in q f r percentages 54 42 4 88 10 2 62 34 4 82 16 2 99 1 0 this sequence is a function of the changing paleoclimate glacial arid temperate humid warm semiarid warm semihumid warm humid associated with the overall global limatic change and the changing latitudinal location of india as it moved from a location close to the south pole northward during the interval of time represented by gondwana deposition lithologic and fossil evidence indicates that during deposition of the fountain formation climate gradually changed from relatively wet and warm to at least semiarid and warm this climatic change is the main factor responsible for the difference in compositional maturity of the older fountain sandstone q f r 65 26 9 relative to the younger q f r 54 36 10 cutler sandstone was deposited in an arid climate and has maximum compositional immaturity q f r 49 44 7 a log log plot of the ratio of total quartz to total feldspar plus rock fragments against the ratio of total polycrystalline quartz to to al feldspar plus rock fragments is a sensitive discriminator of first cycle sandstones with differing climatic heritage bulk chemical composition data support interpretation of climate from framework mineralogy but by themselves are not sensitive enough to be unequivocal indicators the same is true for the ztr index and observations of degree and abundance of solution pits embayments on detrital quartz results of this study support the conclusion of earlier empirical and theoretical analyses which suggest that the optimum conditions for the production and preservation of a distinctive climatic signature on sand composition are met in extensional plate tectonic settings such settings are characterized by coarse grained crystalline parent rocks short transport distances in low order streams deposition in nonroutine environments and shallow burial diagenesis however even in such optimum settings only rarely will climate be a more important determinant of sandstone composition than the tectonic setting itself

palaeozoic mesozoic and contemporaneous megaspores from the tertiary of southern england indicators of sedimentary provenance and ancient vegetation megaspores are recorded from several localities in the thanet and reading beds upper palaeocene of southern england evidence from their taxonomic affinity colour compression and state of preservation indicates three separate groups two reworked from lower westphalian coal measures and lower cretaceous sequences respectively and one of contemporaneous forms mesozoic megaspores redeposited in the british tertiary have not previously been recorded isolated carboniferous forms have been found before but their possible value in provenance studies has not been fully explored the new material includes a diverse assemblage of numerous westphalian and lower cretaceous forms from a single horizon in the thanet beds of possible source areas it seems most likely that these spores were reworked from the moray firth basin and elsewhere near the present eastern coast of scotland by means of longshore drift following a roughly n s palaeocoastline evidence from other studies of heavy mineral assemblages and palaeogeography also indicates derivation of thanet beds sediments by this means the cenozoic megaspores include an azolla water fern and two distinct lycopsid forms one probably of selaginellalean and one of isoetalean affinity these latter provide the first clear evidence of lycopsids in british tertiary floras and expand our knowledge of freshwater macrophytes of that time the assemblages of minerisporites show morphological trends which may prove to be of biostratigraphic value

palaeogeographical and palaeoclimatic evolution of the intracratonic parnaiba basin ne brazil using gplates plate tectonic reconstructions and chemostratigraphic tools the intracratonic paleozoic parnaiba basin ne brazil covers c 600 000 km 2 lithostratigraphic mineralogical and geochemical approaches were used to deduce the changing environments of deposition of the parnaiba basin based on 150 mostly argillite samples silurian earliest triassic the results indicate that brackish conditions and mixed kerogen sources were important and that the palaeowater depth and sources of meteoric water varied from west to east the chemical index of alteration results together with other elemental and mineralogical data indicate that the western margin was humid until the end of the carboniferous and then became arid whereas the eastern margin showed the reverse pattern this supports observations of major stratigraphic differences between the two margins there were also major variations in the palaeoredox and palaeosalinity conditions oxic and fresh to brackish water systems in the east v suboxic to dysoxic and saline water systems in the west the clay mineral assemblages are characteristically dominated by kaolinite which is considered to be inherited from weathering horizons developed in silicic granitic terranes regional palaeoclimate reconstructions were carried out in the parnaiba basin by linking gplates software with the south america plate motion model northwards drift during the paleozoic coupled with a latest paleozoic rotation phase and with the regional database of information on the location formation name and original references for each locality

formation of pyrite in freshwater sediments implications for c s ratios abstract the vertical distribution of pyrite acid volatile sulphide avs carbon and total s st were determined directly in the sediments of three lakes of different trophic status the results showed that freshwater pyrite formation reflects the redox status of the sediment or overlying waters it appears to form chiefly in reducing sediments which are subject to oxidizing influences by either a low turnover of organic carbon or periodic incursions of oxygen although there are high concentrations of avs in the near surface sediments of productive lakes very little is diagenetically converted to pyrite the feasibility of using sulphur ratios to diagnose whether rocks were formed in marine or freshwater environments is assessed new values for fes 2 fes of 0 5 5 show that this ratio does not provide a reliable test values of c sp where sp represents pyrite sulphur lie within the range of 160 700 and are much higher than previously measured ratios of c s t of 1 50 these new determinations show that if pyrite sulphur is unequivocally measured c s ratios may be a more sensitive indicator of salinity than had been previously thought

propagation pathways of classical labrador sea water from its source region to 26 n 1 more than two decades of hydrography on the abaco line east of the bahamas at 26 n reveals decadal variability in the salinity of classical labrador sea water clsw despite the long distance from its source region in the north atlantic ocean hydrographic time series from the labrador sea and from the abaco line show a pronounced step like decrease in salinity between 1985 and 1995 in the labrador sea and between 1995 and 2010 at the abaco line suggesting a time lag between the two locations of approximately 9 years the amplitude of the anomaly at the abaco line is 50 of the amplitude in the labrador sea a similar time lag and reduction of amplitude is found in the high resolution ofes model in which salinity anomalies can be observed propagating through the deep western boundary current as well as through a broad interior pathway on its way south to the abaco line the clsw becomes 8 standard deviations saltier due to isopycnal mixing with mediterranean outflow water mow climatological data in the north atlantic suggests that the mixing ratio of mow to clsw at the abaco line is 1 4 and that no variability in mow is required to explain the observed variability at the abaco line the data studied here suggest that decadal clsw anomalies stay relatively coherent while getting advected despite the important role of interior pathways

contrasting mineralogical geochemical signatures of manganese oxides guides to metallogenesis manganese oxides have a strong sorption capacity for cationic species in the depositional fluid this sorption capacity is reflected in the chemistry of the oxides which is different from that for the oxides precipitated from fresh water seawater and hydrothermal fluids characteristic mineralogical geochemical enrichment and geochemical association defined statistically signatures in conjunction with several diagnostic plots can be employed to distinguish between the following different genetic types of manganese deposits supergene hydrothermal supergene marine terrestrial supergene dubhite and supergene marine hydrothermal sedimentary exhalative sedex dubhites are manganese oxides derived from the weathering of a mineralized sequence i e base and precious ore minerals and gangue hydrothermal deposits characteristically show geochemical enrichments in the assemblage as ba cu li mo pb sb sr v zn and an mn as geochemical association supergene marine deposits show na k ca mg sr and co cu ni geochemical enrichments whereas supergene terrestrial deposits tend to have very high ba contents and an mn ba association dubhites contain high levels of pb and zn show an mn pb base metal association and are composed of significant quantities of base metal bearing manganese oxides bixbyite braunite hausmannite huebnerite jacobsite and pyrochroite occur predominantly only in hydrothermal deposits whereas chalcophanite coronadite crednerite delta mno 2 groutite hollandite lithiophorite manganite nsutite quenselite ramsdellite romanechite todorokite and woodruffite are generally of supergene origin in addition to established plots presently used to identify marine sedimentary exhalative deposits the following diagnostic graphs were devised na vs mg distinguishes deposits of marine and fresh water origin co ni vs as cu mo pb v zn discriminates between hydrothermal and supergene oxides and pb vs zn can be used to recognize dubhites and therefore prospective sequences

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

brasiliano crustal structure and the tectonic setting of the parnaiba basin of ne brazil results of a deep seismic reflection profile a 1430 km deep crustal seismic reflection profile of the parnaiba basin shows an asymmetric structured western margin and a gently dipping eastern margin the 3 km thick phanerozoic sedimentary section overlies a pronounced planar regional unconformity that crosses three precambrian blocks with differing seismic facies the amazonian araguaia block the parnaiba block and the borborema block the blocks are separated by steep crustal scale boundaries across which seismic facies change abruptly in the west the ophiolitic metasedimentary rocks of the araguaia group overlie the amazonian craton both craton and metasediments terminate eastward against a steep east dipping fault zone defining the amazonian araguaia block and parnaiba block boundary reactivation of this neoproterozoic margin in the late triassic and late jurassic early cretaceous folded and elevated basement and basin over 2 km a second crustal boundary defines the eastern margin of the parnaiba block with the neoproterozoic borborema block this boundary is interpreted as the extension of the transbrasiliano shear zone these data demonstrate that the basement of the parnaiba basin was formed during brasiliano orogenesis by west directed collision related thrusting succeeded by lateral accretion along steep crustal scale boundaries after formation of a post brasiliano peneplain the parnaiba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline no extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

paleoenvironmental implications of novel c30 steranes in precambrian to cenozoic age petroleum and bitumen petroleums and bitumens from early proterozoic 1800 ma to miocene 15 ma age marine strata contain 24 isopropylcholestanes a novel group of c30 steroids the abundance of these compounds relative to 24 n propylcholestanes varies with source rock age late proterozoic vendian and early cambrian oils and or bitumens from siberia the urals oman australia and india have a high ratio of 24 isopropylcholestanes to 24 n propylcholestanes 1 while younger and older samples have a lower ratio 0 4 temporal changes in this parameter may reflect the relative abundance of certain porifera sponges and certain marine algae through time geochemical indicators such as this which can constrain the source rock age of a migrated oil are useful in source rock identification during petroleum exploration

dynamic palaeogeographic reconstructions of the wuchiapingian stage lopingian late permian for the south china block abstract as an important tool in natural resource exploration palaeogeography provides a portrayal of deep time earth the south china block located in the eastern palaeotethys includes multiple natural resources e g coal shale gas bauxite etc in the permian which are well known for their important economic significance in this paper data gathered from literature amount to 431 stratigraphic sections of wuchiapingian age from south china these data were quantitatively analyzed and dynamically reconstructed in the global context under the revised plate motion model by gplates software for the first time a schematic 3 d model was constructed based on the high resolution reconstructions in this paper showing two relatively large highlands developed in the south china block during the wuchiapingian including the kangdian highland in the west and the cathaysia highland in the east and a small yunkai highland in the south a broad platform dominated depositional system with small uplifts and depressions was developed between these two highlands these reconstructions show great potential to explore enriched coal seams and shale gas in the peripheral areas of the two ancient highlands in the west and east of the south china block based on the dynamic evolution of this depositional system south china experienced an overall sea level rise with frequent secondary transgressions and regressions during the wuchiapingian this is in contrast to the widespread continental and evaporite deposits on much of pangea and its surrounding margins after the end guadalupian global regression compilation of conodont biostratigraphic data suggest that the temporal and spatial constraints on the unconformity across the guadalupian lopingian boundary glb resulted from dongwu uplift movement the regression began during the jinogondolella shannoni zone and ended in the clarkina leveni zone but the response time varies in different localities of south china these results provide detailed information about sea level eustacy tectonic uplift sedimentary hiatus resource accumulation and palaeoclimate change from the south china block during the wuchiapingian

a paleoclimatic review of southern south america during the late paleozoic a record from icehouse to extreme greenhouse conditions abstract this paper provides a review of the late mississippian to permian paleoclimatic history for southern south america based on lithologic indicators biostratigraphic information and chronostratigraphic data the region is divided into three major types of basins 1 eastern intraplate basins e g parana basin 2 western retroarc basins e g paganzo basin and 3 western arc related basins e g rio blanco basin four major types of paleoclimatic stages are recognized in these basins 1 glacial late visean early bashkirian 2 terminal glacial bashkirian earliest cisuralian 3 postglacial cisuralian early guadalupian and 4 semiarid arid late guadalupian lopingian the glacial stage began in the late visean and continued until the latest serpukhovian or early bashkirian in almost all of the basins in southern south america during the bashkirian earliest cisuralian terminal glacial stage glacial deposits disappeared almost completely in the western retroarc basins e g paganzo basin but glaciation persisted in the eastern basins e g parana and sauce grande basins a gradual climatic amelioration postglacial stage began to occur during the earliest permian when glacial deposits completely disappeared across all of south america during this interval glacial diamictites were replaced by thick coal beds in the parana basin while north south climatic belts began to be delineated in the western basins which were likely controlled by the distribution of mountain belts along the panthalassan margin of south america towards the late permian climatic belts became less evident and semiarid or arid conditions dominated in the southern south america basins eolian dunes playa lake deposits and mixed eolian fluvial sequences occur in the parana basin and in the western retroarc basins volcanism and volcaniclastic sedimentation dominated along the western margin of south america at that time the stratigraphic record obtained in southern south america supports a long duration transition from icehouse to extreme greenhouse conditions

mean atlantic meridional overturning circulation across 26 5 n from eddy resolving simulations compared to observations 1 observations along 26 5 n are used to examine the time mean structure of the atlantic meridional overturning circulation amoc in eddy resolving simulations with the hybrid coordinate ocean model hycom the model results yield a 5 year mean amoc transport of 18 2 sv compared to 18 4 sv based on data the modeled northward limb of the amoc has a vertical structure similar to observations the southward limb is shallower than observed but deeper than other ocean general circulation models and includes a secondary transport maximum near 4000 m corresponding to nordic seas overflow water the modeled flow through the florida strait and the deep western boundary current dwbc east of abaco bahamas are also approximately consistent with observations the model results are used to clarify the sources of the northward amoc transport and to explore the circulation pattern of the southward transport in the western subtropical north atlantic in the range 18 33 n about 14 1 sv of the modeled northward amoc transport is through the florida strait and the remainder through the mid ocean primarily in the ekman layer but also below 600 m the modeled amoc transport is about 2 3 surface water and 1 3 antarctic intermediate water with no contribution from the thermocline water in between in the western subtropical north atlantic the model results depict a complicated deep circulation pattern associated with the complex bathymetry the dwbc flows southward then eastward in both the upper and lower north atlantic deep water nadw layers but with different offshore recirculation pathways and there exists a second more northern branch of eastward flow in the lower nadw layer

variability in deep western boundary current transports preliminary results from 26 5 n in the atlantic 1 transport fluctuations of the deep limb of the meridional overturning circulation moc near the western boundary are presented from a line of inverted echo sounders bottom pressure sensors and a deep current meter east of abaco island bahamas at 26 5 n from september 2004 through september 2005 the mean southward flow between 800 dbar and 4800 dbar was 39 106 m3 s 1 with a northward recirculation of 28 106 m3 s 1 leaving a net southward flow of 11 106 m3 s 1 as the through flow of the deep western boundary current dwbc the mean southward dwbc flow essentially equals previous values that were measured at the same location by arrays of current meters deployed from 1986 to 1992 dwbc transport spectra indicate that barotropic and baroclinic changes have very similar energy levels at most periods less than 10 days and that barotropic changes dominate at periods of 10 80 days

western boundary current structure and variability east of abaco bahamas at 26 5 n abstract during april 1987 to june 1988 a four mooring transport resolving current meter array was deployed in a section extending 70 km eastward from abaco bahamas at 26 5 n mean currents in the upper layer 800 m showed a clockwise rotation that appears to be associated with a small scale quasi permanent anticyclonic gyre centered just northeast or the bahamas deep mean currents were persistenly southward and indicated a strong deep jet the deep western boundary current dwbc reaching core speeds of 20 cm s 1at 2500 m depth about 25 km seaward of the western boundary vertically integrated meridional volume transport over the section showed a surprisingly large variability ranging from approximately 20 sv northward to 70 sv southward 1 sv 106m3s 1 the mean meridional volume transport was 30 sv to the south with about 3 sv flowing northward above 800 m which could be produced by a weak antilles current and 33 sv flowing southward below 800 m the deep southward transport of 33 sv i

jaibaras trough an important element in the early tectonic evolution of the parnaiba interior sag basin northern brazil abstract the jaibaras trough jt is a late precambrian to early paleozoic basin that is related to the subsequent formation of the paleozoic mesozoic parnaiba intracratonic basin in northern brazil based on available geological and geophysical data we propose that the jt is a prototypical rift basin formed by brittle reactivation processes along ancient shear zones associated with the ne sw transbrasiliano lineament of the brazilian panafrican orogeny the jt evolution is partially constrained by the stratigraphic sequences outcropping at the border of the basin or drilled by exploratory boreholes the preserved sedimentary column records in a broad sense the sedimentary and tectonic controls on the early subsidence in the parnaiba basin these sequences are constituted by basal fault scarp related paraconglomerates followed laterally and vertically by fluvial lacustrine sandstones interbedded with reddish shale the jt geological evolution is also constrained by petrology of four magmatic events coreau dike swarm mucambo and meruoca plutons and mafic parapui suite related to continental rifting the late precambrian early paleozoic age of jt is perfectly compatible with continental fragmentation during the processes of lithospheric stretching that affected western gondwana at that time the contemporaneous development of a magmatic sedimentary system dated as cambrian to ordovician and confined to the rift trend raised the thermal gradient and produced a mantle softening discontinuity giving rise to a broad uplifted region this might explain the close temporal and spatial association between the transbrasiliano lineament and the early paleozoic depositional axis of the parnaiba basin an integrated interpretation of the jt and the early tectonostratigraphic behaviour of the parnaiba basin indicates that the jt might constitute a precursory stage that was slightly inverted before the initiation of sag deposition in the intracratonic basin

permian paleogeography of west central pangea reconstruction using sabkha type gypsum bearing deposits of parnaiba basin northern brazil abstract extreme aridity during late permian early triassic period was the main factor for resetting the entire paleoclimate of the planet permian evaporite basins and lacustrine red beds were widely distributed along the supercontinent of pangea sulphate deposits in western pangea particularly in northern brazil accumulated in an extensive playa lake system outcrop based facies and stratigraphic analysis of up to 20 m thick evaporite siliciclastic deposits reveal the predominance of laminated reddish mudstone with subordinate limestone marl and lenses of gypsum the succession was deposited in shallow lacustrine and inland sabkha environments associated with saline pans and mudflats gypsum deposits comprise six lithofacies 1 bottom growth gypsum 2 nodular micronodular gypsum 3 mosaic gypsum 4 fibrous prismatic gypsum 5 alabastrine gypsum and 6 rosettes of gypsum gypsum types 1 and 2 are interpreted as primary deposition in saline pans bottom growth gypsum forms grass like crusts while nodular micronodular gypsum indicates displacive precipitation of the crust in shallow water and the groundwater capillary zone types 3 and 4 are early diagenetic precipitates abundant inclusions of tiny lath like anhydrite crystals suggest a primary origin of anhydrite alabastrine gypsum fibrous gypsum satinspar and rosettes of gypsum probably derived from near surface hydration of anhydrite the gypsum bearing deposits in the parnaiba basin contribute towards understanding paleogeographic changes in western pangea a progressive uplift of east pangea culminated in the forced regression and retreat of epicontinental seas to the west restricted seas or large lakes were formed before the definitive onset of desert conditions in pangea leading to the development of extensive ergs

a census of eddies observed in north atlantic sofar float data abstract sofar floats that looped in discrete eddies were studied in order to map and describe the distribution and characteristics of eddies in the north atlantic one hundred eighteen individual looping float trajectories loopers were identified each consisting of two or more consecutive loops each looper was interpreted to be in a discrete eddy and its characteristics were estimated from the float trajectory the highest percentage of loopers occured at 700m in the newfoundland basin where roughly half of the float data were in loopers mostly cyclones in the gulf stream region approximately 20 of the float data recorded at 700m were in loopers again mostly cyclones overall 21 of 700m data and 6 of 2000m data were in loopers the fastest swirl speeds 40cm s 1 were in cyclones south of the gulf stream most of which were in gulf stream rings but numerous swift 35cm s 1 anticyclones were found there too swirl velocity decreased with depth to roughly half as swift at 2000m as at 700m for three eddies in the sargasso sea measured simultaneously with floats at these two depths in the western north atlantic the average swirl velocity of cyclones and anticyclones was the same translation velocity of eddies was generally westward to southwestward at a few cm s 1 the mean translation velocity of 39 eddies in the sargasso sea was u bar 2 8 0 4 cm s 1 v bar 0 4 0 4 cm s 1 many of these were located just south of the gulf stream in the region of its recirculation and were probably advected by a westward current there near the gulf stream and along its extension in the newfoundland basin eddies were often advected downstream with speeds up to 15 20cm s 1 and eddy trajectories were often complicated south of 30 n and near the western boundary 700m eddies were advected northwestward and 1300m and 2000m eddies southeastward by boundary currents there numerous energetic anticyclones were observed south of the gulf stream one was tracked for 430 days and its properties well measured the formation of these eddies has not been documented but they are inferred to have formed near and by the gulf stream and to consist of a thick layer of 18 c water lying above a depression in the thermocline analogous anticyclones were observed in the newfoundland basin seaward of the gulf stream extension there

deep cross equatorial flow in the atlantic measured with sofar floats neutrally buoyant sofar floats at nominal depths of 800 1800 and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the atlantic near 6 n and at several sites near 11 n as well as along the equator trajectories at 1800 m show a swift 50 cm s narrow 100 km wide southward flowing deep western boundary current dwbc extending from 7 n to the equator the average transport per unit depth in the dwbc was estimated to be 13 8 103 m2 s coupling this value with mean velocities measured in the dwbc by current meters gave a volume transport of 15 106 m3 s between depths of 900 m and 2800 m approximately 6 106 m3 s recirculated northward between the dwbc and the mid atlantic ridge leaving 9 106 m3 s as cross equatorial transport no obvious dwbc nor swift equatorial current was observed by the 3300 m floats a low mean velocity at this depth lay between f 11 and higher velocity cores above and below the 1800 m trajectories also suggest that at times february march 1989 the north atlantic deep water in the dwbc turned eastward and flowed along the equator and at other times august september 1990 the dwbc crossed the equator and continued southward the velocity near the equator calculated by grouping floats in a box along the equator was eastward at 4 1 cm s from february 1989 to february 1990 and westward at 4 6 cm s from march 1990 to november 1990 thus the amount of cross equatorial flow in the dwbc appeared to be linked to low frequency variability of the structure of the equatorial current system floats in antarctic intermediate water at 800 m revealed a northwestward western boundary current although flow patterns were complicated three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm s six 800 m floats drifted eastward along the equator between 5 s and 6 n at a mean velocity of 11 cm s one reached 5 w in the gulf of guinea suggesting that the equatorial currents at this depth extended at least 35 40 along the equator three of these floats reversed direction near the end of the tracking period

crustal structure beneath the paleozoic parnaiba basin revealed by airborne gravity and magnetic data brazil abstract the parnaiba basin is a large paleozoic syneclise in northeastern brazil underlain by precambrian crystalline basement which comprises a complex lithostructural and tectonic framework formed during the neoproterozoic eopaleozoic brasiliano pan african orogenic collage a sag basin up to 3 5 km thick and 1000 km long formed after the collage the lithologic composition structure and role in the basin evolution of the underlying basement are the focus of this study airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the parnaiba basin results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding neoproterozoic fold belts in the basement triangular shaped basement inliers are geophysically defined in the central region of this continental scale neoproterozoic convergence zone a 3 d gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36 40 5 km deep crustal root with borders defined by a high density and thinner crust forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by brasiliano shear zones representing lithospheric sutures of the amazonian and sao francisco cratons tocantins province and parnaiba block in addition coincident residual gravity residual magnetic and pseudo gravity lows indicate two complex systems of eopaleozoic rifts related to the initial phase of the sag deposition which follow basement trends in several directions